flashinfer.py

# SPDX-License-Identifier: Apache-2.0
# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
"""Attention layer with FlashInfer."""

from dataclasses import dataclass
from functools import partial
from typing import ClassVar

import numpy as np
import torch
from flashinfer import (
    BatchDecodeWithPagedKVCacheWrapper,
    BatchPrefillWithPagedKVCacheWrapper,
    BatchPrefillWithRaggedKVCacheWrapper,
    MultiLevelCascadeAttentionWrapper,
)
from flashinfer.decode import fast_decode_plan, trtllm_batch_decode_with_kv_cache
from flashinfer.prefill import trtllm_batch_context_with_kv_cache
from flashinfer.utils import FP4Tensor
from typing_extensions import override

from vllm import envs
from vllm.config import (
    CUDAGraphMode,
    VllmConfig,
    get_current_vllm_config_or_none,
)
from vllm.config.cache import CacheDType
from vllm.distributed.parallel_state import get_dcp_group
from vllm.logger import init_logger
from vllm.model_executor.layers.batch_invariant import (
    vllm_is_batch_invariant,
)
from vllm.model_executor.layers.quantization.utils.quant_utils import (
    QuantKey,
    kFp8StaticTensorSym,
    kNvfp4Dynamic,
)
from vllm.platforms import current_platform
from vllm.platforms.interface import DeviceCapability
from vllm.triton_utils import tl, triton
from vllm.utils.flashinfer import (
    can_use_trtllm_attention,
    use_trtllm_attention,
)
from vllm.utils.math_utils import cdiv
from vllm.utils.platform_utils import is_pin_memory_available
from vllm.utils.torch_utils import is_strictly_contiguous
from vllm.v1.attention.backend import (
    AttentionBackend,
    AttentionCGSupport,
    AttentionImpl,
    AttentionMetadataBuilder,
    AttentionType,
    CommonAttentionMetadata,
    MultipleOf,
)
from vllm.v1.attention.backends.utils import (
    KVCacheLayoutType,
    get_dcp_local_seq_lens,
    get_kv_cache_layout,
    get_per_layer_parameters,
    infer_global_hyperparameters,
    split_decodes_and_prefills,
)
from vllm.v1.attention.ops.common import cp_lse_ag_out_rs
from vllm.v1.attention.ops.dcp_alltoall import dcp_a2a_lse_reduce
from vllm.v1.attention.ops.merge_attn_states import merge_attn_states
from vllm.v1.kv_cache_interface import AttentionSpec, UniformTypeKVCacheSpecs
from vllm.v1.utils import CpuGpuBuffer

FLASHINFER_WORKSPACE_BUFFER_SIZE_BATCH_INVARIANT = 2048 * 1024 * 1024

FP8_DTYPE = current_platform.fp8_dtype()
FP4_DTYPE = torch.uint8

logger = init_logger(__name__)

trtllm_gen_workspace_buffer = None


def _get_trtllm_gen_workspace_buffer():
    global trtllm_gen_workspace_buffer
    if trtllm_gen_workspace_buffer is None:
        trtllm_gen_workspace_buffer = torch.zeros(
            envs.VLLM_FLASHINFER_WORKSPACE_BUFFER_SIZE, dtype=torch.uint8, device="cuda"
        )
    return trtllm_gen_workspace_buffer


@triton.jit
def _trtllm_prefill_attn_kvfp8_dequant(
    kv_cache_ptr,
    block_tables_prefill_ptr,
    block_table_stride,
    mock_kv_cache_ptr,
    k_scale_ptr,
    v_scale_ptr,
    K_CACHE_STRIDE: tl.constexpr,
    KV_CACHE_STRIDE: tl.constexpr,
):
    batch_idx = tl.program_id(0).to(tl.int64)
    mock_block_table_idx = tl.program_id(1).to(tl.int64)
    orig_page_num = tl.load(
        block_tables_prefill_ptr + batch_idx * block_table_stride + mock_block_table_idx
    ).to(tl.int64)
    if orig_page_num <= 0:
        return
    dequant_dtype = mock_kv_cache_ptr.dtype.element_ty

    # Dequantize K
    k_scale_val = tl.load(k_scale_ptr)
    offset = orig_page_num * KV_CACHE_STRIDE + tl.arange(0, K_CACHE_STRIDE)
    fp8_vals = tl.load(kv_cache_ptr + offset)
    dequantized_vals = fp8_vals.to(tl.float32) * k_scale_val
    mock_cache_offset = (
        batch_idx * block_table_stride + mock_block_table_idx + 1
    ) * KV_CACHE_STRIDE + tl.arange(0, K_CACHE_STRIDE)
    dequantized_vals = dequantized_vals.to(dequant_dtype)
    tl.store(mock_kv_cache_ptr + mock_cache_offset, dequantized_vals)

    # Dequantize V
    v_scale_val = tl.load(v_scale_ptr)
    offset = (
        orig_page_num * KV_CACHE_STRIDE + K_CACHE_STRIDE + tl.arange(0, K_CACHE_STRIDE)
    )
    fp8_vals = tl.load(kv_cache_ptr + offset)
    dequantized_vals = fp8_vals.to(tl.float32) * v_scale_val
    mock_cache_offset = (
        (batch_idx * block_table_stride + mock_block_table_idx + 1) * KV_CACHE_STRIDE
        + K_CACHE_STRIDE
        + tl.arange(0, K_CACHE_STRIDE)
    )
    dequantized_vals = dequantized_vals.to(dequant_dtype)
    tl.store(mock_kv_cache_ptr + mock_cache_offset, dequantized_vals)


def trtllm_prefill_attn_kvfp8_dequant(
    kv_cache: torch.Tensor,
    block_tables_prefill: torch.Tensor,
    k_scale: torch.Tensor,
    v_scale: torch.Tensor,
    dequant_dtype: torch.dtype,
) -> tuple[torch.Tensor, torch.Tensor]:
    batch_size, num_of_page_per_token = block_tables_prefill.shape
    s = kv_cache.shape
    assert s[1] == 2
    assert dequant_dtype in (torch.bfloat16, torch.float16)
    k_cache_stride = s[2] * s[3] * s[4]
    kv_cache_stride = k_cache_stride * s[1]
    new_s = (batch_size * num_of_page_per_token + 1, s[1], s[2], s[3], s[4])
    # mock kv cache contains just the pages needed by this prefill
    mock_kv_cache = torch.empty(new_s, dtype=dequant_dtype, device=kv_cache.device)
    # we simply sequentially index the pages needed by this prefill
    mock_block_table = torch.arange(
        start=1,
        end=batch_size * num_of_page_per_token + 1,
        dtype=torch.int32,
        device=block_tables_prefill.device,
    ).reshape(batch_size, num_of_page_per_token)
    grid = (batch_size, num_of_page_per_token)
    _trtllm_prefill_attn_kvfp8_dequant[grid](
        kv_cache,
        block_tables_prefill,
        num_of_page_per_token,
        mock_kv_cache,
        k_scale,
        v_scale,
        k_cache_stride,
        kv_cache_stride,
    )
    return mock_kv_cache, mock_block_table


class BatchDCPPrefillWrapper:
    def __init__(
        self,
        workspace_buffer: torch.Tensor | None = None,
        dcp_a2a: bool = False,
    ):
        if dcp_a2a:
            self._dcp_combine = partial(dcp_a2a_lse_reduce, is_lse_base_on_e=False)
        else:
            self._dcp_combine = partial(cp_lse_ag_out_rs, is_lse_base_on_e=False)
        self._context = BatchPrefillWithPagedKVCacheWrapper(
            workspace_buffer, get_kv_cache_layout()
        )
        self._new_tokens = BatchPrefillWithRaggedKVCacheWrapper(
            workspace_buffer, get_kv_cache_layout()
        )

    def plan(
        self,
        qo_indptr_cpu: torch.Tensor,
        paged_kv_indptr_cpu: torch.Tensor,
        paged_kv_indices: torch.Tensor,
        paged_kv_last_page_len_cpu: torch.Tensor,
        page_size: int,
        num_qo_heads: int,
        dcp_world_size: int,
        num_kv_heads: int,
        head_dim: int,
        sm_scale: float,
        window_left: int,
        logits_soft_cap: float | None,
        q_data_type: torch.dtype,
        kv_cache_dtype: torch.dtype,
        prefill_fixed_split_size: int,
        disable_split_kv: bool,
    ):
        """Plan the prefill operation with given parameters."""
        self._context.plan(
            qo_indptr=qo_indptr_cpu,
            paged_kv_indptr=paged_kv_indptr_cpu,
            paged_kv_indices=paged_kv_indices,
            paged_kv_last_page_len=paged_kv_last_page_len_cpu,
            num_qo_heads=num_qo_heads * dcp_world_size,
            num_kv_heads=num_kv_heads,
            head_dim_qk=head_dim,
            page_size=page_size,
            causal=False,  # This is context run
            sm_scale=sm_scale,
            window_left=window_left,
            logits_soft_cap=logits_soft_cap,
            q_data_type=q_data_type,
            kv_data_type=kv_cache_dtype,
            fixed_split_size=prefill_fixed_split_size,
            disable_split_kv=disable_split_kv,
        )
        self._new_tokens.plan(
            qo_indptr=qo_indptr_cpu,
            kv_indptr=qo_indptr_cpu,
            num_qo_heads=num_qo_heads,
            num_kv_heads=num_kv_heads,
            head_dim_qk=head_dim,
            head_dim_vo=head_dim,
            causal=True,  # This is newtokens run
            sm_scale=sm_scale,
            window_left=window_left,
            logits_soft_cap=logits_soft_cap,
            q_data_type=q_data_type,
        )

    def run(
        self,
        layer: torch.nn.Module,
        prefill_query: torch.Tensor,
        kv_cache_permute: torch.Tensor,
        key: torch.Tensor,
        value: torch.Tensor,
        out: torch.Tensor,
    ):
        prefill_query_across_dcp = get_dcp_group().all_gather(
            prefill_query.contiguous(), dim=1
        )
        output_context_tmp, lse_context_tmp = self._context.run(
            prefill_query_across_dcp,
            kv_cache_permute,
            k_scale=layer._k_scale_float,
            v_scale=layer._v_scale_float,
            return_lse=True,
        )
        output_context, lse_context = self._dcp_combine(
            output_context_tmp,
            lse_context_tmp,
            get_dcp_group(),
            return_lse=True,
        )
        lse_context = lse_context.transpose(0, 1).contiguous()

        output_query, lse_query = self._new_tokens.run(
            prefill_query,
            key,
            value,
            return_lse=True,
        )
        lse_query = lse_query.transpose(0, 1).contiguous()

        merge_attn_states(
            out,
            output_context,
            lse_context,
            output_query,
            lse_query,
        )
        return out


class FlashInferBackend(AttentionBackend):
    accept_output_buffer: bool = True
    supported_dtypes: ClassVar[list[torch.dtype]] = [torch.float16, torch.bfloat16]
    supported_kv_cache_dtypes: ClassVar[list[CacheDType]] = [
        "auto",
        "bfloat16",
        "fp8",
        "fp8_e4m3",
        "fp8_e5m2",
    ]

    @staticmethod
    def get_supported_kernel_block_sizes() -> list[int | MultipleOf]:
        # Note: Not sure for all platforms, but on Blackwell,
        # only support a page size of 16, 32, 64.
        return [16, 32, 64]

    @staticmethod
    def get_name() -> str:
        return "FLASHINFER"

    @staticmethod
    def get_impl_cls() -> type["FlashInferImpl"]:
        return FlashInferImpl

    @staticmethod
    def get_builder_cls() -> type["FlashInferMetadataBuilder"]:
        return FlashInferMetadataBuilder

    @staticmethod
    def get_kv_cache_shape(
        num_blocks: int,
        block_size: int,
        num_kv_heads: int,
        head_size: int,
        cache_dtype_str: str = "auto",
    ) -> tuple[int, ...]:
        return (num_blocks, 2, block_size, num_kv_heads, head_size)

    @staticmethod
    def get_kv_cache_stride_order(
        include_num_layers_dimension: bool = False,
    ) -> tuple[int, ...]:
        # `stride_order` indicates the permutation that gets us from
        # `get_kv_cache_shape` to the actual memory layout we want.
        cache_layout = get_kv_cache_layout()
        if cache_layout == "NHD" and include_num_layers_dimension:
            # (num_blocks, num_layers, 2, block_size, num_kv_heads, head_size)
            return (1, 0, 2, 3, 4, 5)
        elif cache_layout == "NHD":
            stride_order = (0, 1, 2, 3, 4)
        elif cache_layout == "HND" and include_num_layers_dimension:
            # (num_blocks, 2, num_kv_heads, num_layers, block_size, head_size)
            return (1, 2, 4, 0, 3, 5)
        elif cache_layout == "HND":
            stride_order = (0, 1, 3, 2, 4)
        else:
            raise ValueError(f"Unknown cache layout format {cache_layout}.")
        return stride_order

    @staticmethod
    def get_fp8_dtype_for_flashinfer(kv_cache_dtype: str) -> torch.dtype:
        if kv_cache_dtype in ("fp8", "fp8_e4m3"):
            return torch.float8_e4m3fn
        elif kv_cache_dtype == "fp8_e5m2":
            return torch.float8_e5m2
        else:
            raise ValueError(f"Unrecognized FP8 dtype: {kv_cache_dtype}")

    @classmethod
    def get_supported_head_sizes(cls) -> list[int]:
        # https://github.com/flashinfer-ai/flashinfer/blob/3d55c71a62052c590c130897d3a3db49b14fcc34/include/flashinfer/utils.cuh#L157
        return [64, 128, 256]

    @classmethod
    def supports_compute_capability(cls, capability: DeviceCapability) -> bool:
        return capability >= DeviceCapability(7, 5) and capability <= DeviceCapability(
            12, 1
        )

    @classmethod
    def supports_sink(cls) -> bool:
        """FlashInfer supports sinks when TRTLLM attention is available (SM100)."""
        from vllm.utils.flashinfer import (
            force_use_trtllm_attention,
            supports_trtllm_attention,
        )

        # Respect explicit disable flag (e.g.,
        # --attention-config.use_trtllm_attention=0)
        if force_use_trtllm_attention() is False:
            return False

        # Check if TRTLLM is supported on this platform
        return supports_trtllm_attention()

    @classmethod
    def get_required_kv_cache_layout(cls) -> KVCacheLayoutType | None:
        capability = current_platform.get_device_capability()
        if capability is not None and capability.major == 10:
            return "HND"
        return None

    forward_includes_kv_cache_update: bool = False


@dataclass
class FIPrefill:
    """Metadata for the native FlashInfer prefill pathway (non-TRTLLM)."""

    wrapper: BatchPrefillWithPagedKVCacheWrapper | BatchDCPPrefillWrapper


@dataclass
class FIDecode:
    """Metadata for the native FlashInfer decode pathway (non-TRTLLM)."""

    wrapper: BatchDecodeWithPagedKVCacheWrapper


@dataclass
class TRTLLMPrefill:
    """Metadata for the TRTLLM prefill pathway."""

    block_tables: torch.Tensor
    """
    The slice of the block table tensor corresponding *only* to prefill requests.
    Shape: [num_prefills, max_num_blocks_per_seq]
    """

    seq_lens: torch.Tensor
    """
    The slice of the sequence lengths tensor corresponding *only* to prefill requests.
    Shape: [num_prefills]
    """

    cum_seq_lens_q: torch.Tensor
    cum_seq_lens_kv: torch.Tensor

    max_q_len: int
    """
    The maximum query length *among prefill requests*.
    """

    max_seq_len: int
    """The maximum sequence length for KV Cache."""


@dataclass
class TRTLLMDecode:
    """Metadata for the TRTLLM decode pathway."""

    block_tables: torch.Tensor
    """
    The slice of the block table tensor corresponding *only* to decode requests.
    Shape: [num_decodes, max_num_blocks_per_seq]
    """

    seq_lens: torch.Tensor
    """
    The slice of the sequence lengths tensor corresponding *only* to decode requests.
    Shape: [num_decodes]
    """

    max_seq_len: int
    """The maximum sequence length for KV Cache."""


@dataclass
class FlashInferMetadata:
    num_actual_tokens: int
    """Total number of tokens in the batch (excluding padding)."""

    slot_mapping: torch.Tensor
    """Tensor for writing K/V to the cache. Shape: [num_actual_tokens]"""

    q_data_type: torch.dtype

    num_decodes: int
    num_decode_tokens: int
    num_prefills: int
    num_prefill_tokens: int

    prefill: FIPrefill | TRTLLMPrefill | None
    """
    Holds the metadata for the prefill portion of the batch.
    Will be `None` if `num_prefill_tokens == 0`.
    """

    decode: FIDecode | TRTLLMDecode | None
    """
    Holds the metadata for the decode portion of the batch.
    Will be `None` if `num_decode_tokens == 0`.
    """

    # --- Special Case: Cascade Attention ---

    use_cascade: bool
    """
    If True, the entire batch is a cascade attention call, and the
    `prefill` and `decode` fields will both be None.
    """

    cascade_wrapper: MultiLevelCascadeAttentionWrapper | None


class FlashInferMetadataBuilder(AttentionMetadataBuilder[FlashInferMetadata]):
    reorder_batch_threshold: int = 1

    def __init__(
        self,
        kv_cache_spec: AttentionSpec,
        layer_names: list[str],
        vllm_config: VllmConfig,
        device: torch.device,
    ):
        super().__init__(kv_cache_spec, layer_names, vllm_config, device)
        self.cache_config = vllm_config.cache_config
        self.model_config = vllm_config.model_config
        self.attention_config = vllm_config.attention_config
        self._workspace_buffer = None
        self._prefill_wrapper: (
            BatchPrefillWithPagedKVCacheWrapper | BatchDCPPrefillWrapper | None
        ) = None  # Wrapper for prefill/append
        self._decode_wrapper = None  # Wrapper for decode (general shape)

        if vllm_is_batch_invariant():
            self.decode_fixed_split_size = 2048
            self.prefill_fixed_split_size = 4096
            self.disable_split_kv = True
        else:
            self.decode_fixed_split_size = -1
            self.prefill_fixed_split_size = -1
            self.disable_split_kv = False

        self.compilation_config = vllm_config.compilation_config
        max_num_pages_per_req = cdiv(
            self.model_config.max_model_len, self.kv_cache_spec.block_size
        )
        max_num_reqs = vllm_config.scheduler_config.max_num_seqs
        max_num_pages = max_num_reqs * max_num_pages_per_req
        speculative_config = vllm_config.speculative_config
        num_spec_tokens = (
            speculative_config.num_speculative_tokens
            if speculative_config is not None
            else 0
        )
        self.enable_cuda_graph = (
            self.compilation_config.cudagraph_mode.decode_mode() == CUDAGraphMode.FULL
        )
        if self.enable_cuda_graph:
            # For full cudagraph capture, one `decode_wrapper` for each batch
            # size is needed for FlashInfer.
            self._decode_wrappers_cudagraph: dict[
                int, BatchDecodeWithPagedKVCacheWrapper
            ] = {}
            self._decode_cudagraph_max_bs = (1 + num_spec_tokens) * max_num_reqs
            if self.compilation_config.max_cudagraph_capture_size is not None:
                self._decode_cudagraph_max_bs = min(
                    self._decode_cudagraph_max_bs,
                    self.compilation_config.max_cudagraph_capture_size,
                )
        try:
            self.dcp_world_size = get_dcp_group().world_size
            self.dcp_rank = get_dcp_group().rank_in_group
            self.dcp_kv_cache_interleave_size = (
                vllm_config.parallel_config.dcp_kv_cache_interleave_size
            )
        except AssertionError:
            # DCP might not be initialized in testing
            self.dcp_world_size = 1
            self.dcp_rank = 0
            self.dcp_kv_cache_interleave_size = 1
        self.use_dcp = self.dcp_world_size > 1
        self.dcp_a2a = (
            self.use_dcp and vllm_config.parallel_config.dcp_comm_backend == "a2a"
        )

        self.num_qo_heads = self.model_config.get_num_attention_heads(
            self.vllm_config.parallel_config
        )

        self.num_kv_heads = self.kv_cache_spec.num_kv_heads
        self.head_dim = self.kv_cache_spec.head_size
        self.page_size = self.kv_cache_spec.block_size

        self.cache_dtype = self.cache_config.cache_dtype
        if self.cache_dtype.startswith("fp8"):
            self.kv_cache_dtype = FlashInferBackend.get_fp8_dtype_for_flashinfer(
                self.cache_dtype
            )
        else:
            assert self.kv_cache_spec.dtype == self.model_config.dtype
            self.kv_cache_dtype = self.kv_cache_spec.dtype

        # Use model dtype as q dtype when TRTLLM attn is not supported, or
        # --attention-config.disable_flashinfer_q_quantization is set to 1. Otherwise,
        # try to use fp8 q if kv cache is fp8, and will fall back to model dtype
        # if TRTLLM attention kernel is not used when building attn metadata
        can_use_trtllm = can_use_trtllm_attention(self.num_qo_heads, self.num_kv_heads)
        if (
            can_use_trtllm
            and not vllm_config.attention_config.disable_flashinfer_q_quantization
        ):
            self.q_data_type = self.kv_cache_dtype
        else:
            self.q_data_type = self.model_config.dtype

        # Prefer TRTLLM attention for decoding in all cases.
        # This allows us to use AttentionCGSupport.UNIFORM_BATCH mode.
        self.use_trtllm_decode_attention = can_use_trtllm
        self._init_reorder_batch_threshold(1, supports_spec_as_decode=can_use_trtllm)

        self._cascade_wrapper = None  # Wrapper for cascade attention

        # Global hyperparameters shared by all attention layers
        # TODO: discard this for trtllm-gen backend
        self.global_hyperparameters = infer_global_hyperparameters(
            get_per_layer_parameters(vllm_config, layer_names, FlashInferImpl)
        )
        self.sm_scale = self.global_hyperparameters.sm_scale
        self.window_left = self.global_hyperparameters.window_left
        self.logits_soft_cap = self.global_hyperparameters.logits_soft_cap
        self.has_sinks = self.global_hyperparameters.has_sinks
        if self.has_sinks and not can_use_trtllm:
            raise NotImplementedError(
                "FlashInfer backend currently does not support attention "
                "sinks, please use trtllm on blackwell or flash attention on "
                "earlier GPUs."
            )
        # Preparing persistent buffers
        # Since we do not have explicit synchronization in ModelRunnerV2, we do not pin
        # reused CPU buffers to avoid a race condition between step N async copies to
        # GPU and step N+1 buffer updates.
        self.pin_memory = (
            not envs.VLLM_USE_V2_MODEL_RUNNER and is_pin_memory_available()
        )
        self.paged_kv_indptr = self._make_buffer(max_num_reqs + 1)
        self.paged_kv_indptr_cpu_buffer = torch.zeros_like(
            self.paged_kv_indptr.cpu, pin_memory=self.pin_memory
        )  # Extra buffer for mutable paged_kv_indptr.cpu in cuda graph mode
        self.paged_kv_indices = self._make_buffer(max_num_pages)
        self.paged_kv_last_page_len = self._make_buffer(max_num_reqs)

        if self.head_dim == 256 and current_platform.is_device_capability_family(100):
            # https://github.com/flashinfer-ai/flashinfer/issues/1993 reports that
            # head size 256 and block size 16 is not supported on blackwell.
            assert kv_cache_spec.block_size != 16, (
                "There is a bug in FlashInfer "
                "block_size 16 head size 256 support. Please avoid this combination by "
                "passing --block-size 32 or --block-size 64."
            )

    def _make_buffer(
        self, *size: int | torch.SymInt, dtype: torch.dtype = torch.int32
    ) -> CpuGpuBuffer:
        return CpuGpuBuffer(
            *size,
            dtype=dtype,
            device=self.device,
            pin_memory=self.pin_memory,
            with_numpy=True,
        )

    @override  # type: ignore[misc]
    @classmethod
    def get_cudagraph_support(
        cls: type["FlashInferMetadataBuilder"],
        vllm_config: VllmConfig,
        kv_cache_spec: AttentionSpec,
    ) -> AttentionCGSupport:
        """Get the cudagraph support level for FlashInfer attention.

        This depends on whether we can use TRTLLM attention for decodes, since we can
        only do UNIFORM_SINGLE_TOKEN_DECODE if it is unavailable.
        To check this, we must call can_use_trtllm_attention with the number of KV
        heads from the kv_cache_spec. We check all available KV cache specs and
        only return UNIFORM_BATCH if all of them support TRTLLM attention.
        """
        # For UniformTypeKVCacheSpecs, check all contained specs
        kv_specs = (
            kv_cache_spec.kv_cache_specs.values()
            if isinstance(kv_cache_spec, UniformTypeKVCacheSpecs)
            else [kv_cache_spec]
        )
        num_qo_heads = vllm_config.model_config.get_num_attention_heads(
            vllm_config.parallel_config
        )
        has_trtllm_support: bool = len(kv_specs) > 0
        for spec in kv_specs:
            if not isinstance(spec, AttentionSpec):
                # FlashInfer only applies to attention, so we don't consider other types
                # of KV spec (e.g. Mamba) here. This is mostly for type checking.
                continue
            if not can_use_trtllm_attention(
                num_qo_heads=num_qo_heads,
                num_kv_heads=spec.num_kv_heads,
            ):
                has_trtllm_support = False
                break

        if has_trtllm_support:
            return AttentionCGSupport.UNIFORM_BATCH
        else:
            return AttentionCGSupport.UNIFORM_SINGLE_TOKEN_DECODE

    def _get_workspace_buffer(self):
        if self._workspace_buffer is None:
            buffer_size = envs.VLLM_FLASHINFER_WORKSPACE_BUFFER_SIZE
            if vllm_is_batch_invariant():
                buffer_size = FLASHINFER_WORKSPACE_BUFFER_SIZE_BATCH_INVARIANT
            self._workspace_buffer = torch.zeros(
                buffer_size, dtype=torch.uint8, device=self.device
            )
        return self._workspace_buffer

    def set_workspace_buffer(self, workspace_buffer: torch.Tensor):
        self._workspace_buffer = workspace_buffer

    def _get_prefill_wrapper(
        self,
    ) -> BatchPrefillWithPagedKVCacheWrapper | BatchDCPPrefillWrapper:
        if self._prefill_wrapper is None:
            if self.use_dcp:
                self._prefill_wrapper = BatchDCPPrefillWrapper(
                    workspace_buffer=self._get_workspace_buffer(),
                    dcp_a2a=self.dcp_a2a,
                )
            else:
                self._prefill_wrapper = BatchPrefillWithPagedKVCacheWrapper(
                    self._get_workspace_buffer(), get_kv_cache_layout()
                )
        assert self._prefill_wrapper is not None
        return self._prefill_wrapper

    def _get_decode_wrapper(self, batch_size: int, use_cudagraph: bool = False):
        if use_cudagraph:
            decode_wrapper = self._decode_wrappers_cudagraph.get(batch_size, None)
        else:
            decode_wrapper = self._decode_wrapper

        if decode_wrapper is None:
            if use_cudagraph:
                paged_kv_indptr = self.paged_kv_indptr.gpu[: batch_size + 1]
                paged_kv_indices = self.paged_kv_indices.gpu
                paged_kv_last_page_len = self.paged_kv_last_page_len.gpu[:batch_size]
            else:
                paged_kv_indptr = None
                paged_kv_indices = None
                paged_kv_last_page_len = None
            decode_wrapper = BatchDecodeWithPagedKVCacheWrapper(
                self._get_workspace_buffer(),
                get_kv_cache_layout(),
                use_cuda_graph=use_cudagraph,
                paged_kv_indptr_buffer=paged_kv_indptr,
                paged_kv_indices_buffer=paged_kv_indices,
                paged_kv_last_page_len_buffer=paged_kv_last_page_len,
                # Tensor cores are enabled by default because the perf would be
                # at least as good as cuda cores for all attention ops in latest
                # gpus.
                use_tensor_cores=True,
            )

            # save the decode wrapper
            if use_cudagraph:
                self._decode_wrappers_cudagraph[batch_size] = decode_wrapper
            else:
                self._decode_wrapper = decode_wrapper

        return decode_wrapper

    def _get_cascade_wrapper(self):
        if self._cascade_wrapper is None:
            self._cascade_wrapper = MultiLevelCascadeAttentionWrapper(
                2, self._get_workspace_buffer(), get_kv_cache_layout()
            )
        return self._cascade_wrapper

    def _compute_flashinfer_kv_metadata(
        self,
        num_blocks_np: np.ndarray,
        seq_lens_np: np.ndarray,
        block_table_tensor: torch.Tensor,
        num_reqs: int,
        page_size: int,
    ) -> torch.Tensor:
        """
        Compute paged_kv_indptr, paged_kv_indices, paged_kv_last_page_len for FlashInfer
        attention.

        Results are stored in self.paged_kv_indptr,
        self.paged_kv_indices, self.paged_kv_last_page_len buffers.

        Returns paged_kv_indices, a GPU tensor with shape [num_actual_pages].
        """
        # write self.paged_kv_indptr_cpu inplace (0-index is always 0)
        np.cumsum(
            num_blocks_np,
            dtype=np.int32,
            out=self.paged_kv_indptr.np[1 : num_reqs + 1],
        )
        # NOTE(woosuk): Because self.paged_kv_indptr_cpu can be modified
        # after this line (e.g., for cuda graphs), we need to copy the data to
        # self.paged_kv_indptr_buffer to avoid race condition.
        self.paged_kv_indptr_cpu_buffer[: num_reqs + 1] = self.paged_kv_indptr.cpu[
            : num_reqs + 1
        ]
        paged_kv_indptr = self.paged_kv_indptr.gpu[: num_reqs + 1]
        paged_kv_indptr.copy_(
            self.paged_kv_indptr_cpu_buffer[: num_reqs + 1], non_blocking=True
        )

        # write self.paged_kv_indices inplace
        num_actual_pages = self.paged_kv_indptr.np[num_reqs]
        paged_kv_indices = self.paged_kv_indices.gpu[:num_actual_pages]
        _copy_page_indices_kernel[(num_reqs,)](
            paged_kv_indices,
            block_table_tensor,
            block_table_tensor.stride(0),
            paged_kv_indptr,
            BLOCK_SIZE=1024,
        )

        # write self.paged_kv_last_page_len_cpu inplace
        paged_kv_last_page_len_np = seq_lens_np % page_size
        self.paged_kv_last_page_len.np[:num_reqs] = np.where(
            (paged_kv_last_page_len_np == 0) & (seq_lens_np != 0),
            page_size,
            paged_kv_last_page_len_np,
        )
        self.paged_kv_last_page_len.gpu[:num_reqs].copy_(
            self.paged_kv_last_page_len.cpu[:num_reqs], non_blocking=True
        )
        return paged_kv_indices

    def build(
        self,
        common_prefix_len: int,
        common_attn_metadata: CommonAttentionMetadata,
        fast_build: bool = False,
    ) -> FlashInferMetadata:
        num_reqs = common_attn_metadata.num_reqs
        num_actual_tokens = common_attn_metadata.num_actual_tokens
        num_decodes, num_prefills, num_decode_tokens, num_prefill_tokens = (
            split_decodes_and_prefills(
                common_attn_metadata,
                decode_threshold=self.reorder_batch_threshold,
                require_uniform=True,
            )
        )

        page_size = self.page_size
        max_seq_len = common_attn_metadata.max_seq_len
        seq_lens = common_attn_metadata.seq_lens
        block_table_tensor = common_attn_metadata.block_table_tensor
        qo_indptr = common_attn_metadata.query_start_loc
        qo_indptr_cpu = common_attn_metadata.query_start_loc_cpu

        # Step 1: Decide which dispatch modes to use:
        # - Cascade attention (distinct mode)
        # - Prefill (FI native or TRTLLM)
        # - Decode (FI native or TRTLLM)
        use_cascade = common_prefix_len > 0
        uses_spec_reorder = self.reorder_batch_threshold > 1
        prefill_use_trtllm = use_trtllm_attention(
            self.num_qo_heads,
            self.num_kv_heads,
            num_prefill_tokens,
            max_seq_len,
            self.dcp_world_size,
            self.cache_dtype,
            self.q_data_type,
            is_prefill=True,
            force_use_trtllm=self.attention_config.use_trtllm_attention,
            has_sinks=self.has_sinks,
            has_spec=uses_spec_reorder,
        )
        decode_use_trtllm = (
            self.use_trtllm_decode_attention and self.dcp_world_size <= 1
        )

        all_uses_trtllm = (num_prefills == 0 or prefill_use_trtllm) and (
            num_decodes == 0 or decode_use_trtllm
        )
        is_only_trtllm_decode = num_prefills == 0 and (
            num_decodes > 0 and decode_use_trtllm
        )

        if not all_uses_trtllm:
            if self.has_sinks:
                raise NotImplementedError(
                    "FlashInfer backend currently does not support attention "
                    "sinks, please use trtllm on blackwell or flash attention "
                    "on earlier GPUs."
                )

            if not self.global_hyperparameters.has_same_window_lefts:
                raise ValueError(
                    "Window left is not the same for all layers. "
                    "One potential fix is to set disable_sliding_window=True"
                )

            assert self.global_hyperparameters.has_same_all_params, (
                "FlashInfer backend currently only supports models in which "
                "all layers share the same values for the following "
                "hyperparameters: `window_left`, `logits_soft_cap`, "
                "`sm_scale`."
            )

            # The q quantization is not supported for non-trtllm attention,
            # fall back to model dtype.
            self.q_data_type = self.model_config.dtype

        # Step 2: Initialize the output metadata
        # Leave prefill/decode/cascade_wrapper empty, to be populated
        # case by case depending on the batch contents and backend selection.
        attn_metadata = FlashInferMetadata(
            num_actual_tokens=num_actual_tokens,
            slot_mapping=common_attn_metadata.slot_mapping,
            q_data_type=self.q_data_type,
            num_decodes=num_decodes,
            num_decode_tokens=num_decode_tokens,
            num_prefills=num_prefills,
            num_prefill_tokens=num_prefill_tokens,
            use_cascade=use_cascade,
            prefill=None,
            decode=None,
            cascade_wrapper=None,
        )

        # Guard access to seq_lens_cpu, which may not always be needed
        # and can be expensive to retrieve in async mode.
        needs_seq_lens_cpu = self.use_dcp or use_cascade or not is_only_trtllm_decode
        seq_lens_cpu = common_attn_metadata.seq_lens_cpu if needs_seq_lens_cpu else None
        seq_lens_np = seq_lens_cpu.numpy() if seq_lens_cpu is not None else None
        num_blocks_np = (
            (seq_lens_np + (page_size - 1)) // page_size
            if seq_lens_np is not None
            else None
        )

        # Adjust seq_lens_cpu for DCP
        if self.use_dcp:
            assert seq_lens_cpu is not None
            if num_prefills > 0:
                qo_indptr_prefill_cpu = (
                    qo_indptr_cpu[num_decodes:] - qo_indptr_cpu[num_decodes]
                )
                query_lens_prefill_cpu = (
                    qo_indptr_prefill_cpu[1:] - qo_indptr_prefill_cpu[:-1]
                )
                seq_lens_cpu[num_decodes:] = (
                    seq_lens_cpu[num_decodes:] - query_lens_prefill_cpu
                )

            seq_lens_cpu = get_dcp_local_seq_lens(
                seq_lens_cpu,
                self.dcp_world_size,
                self.dcp_rank,
                self.dcp_kv_cache_interleave_size,
            )

        # Adjust num_block_np for cascade attention
        if use_cascade:
            assert num_blocks_np is not None
            assert common_prefix_len % page_size == 0
            num_common_kv_blocks = common_prefix_len // page_size
            num_blocks_np -= num_common_kv_blocks

        # Compute paged_kv_indices if necessary
        needs_paged_kv_indices = use_cascade or not is_only_trtllm_decode
        if needs_paged_kv_indices:
            assert num_blocks_np is not None
            assert seq_lens_np is not None
            paged_kv_indices = self._compute_flashinfer_kv_metadata(
                num_blocks_np,
                seq_lens_np,
                block_table_tensor,
                num_reqs,
                page_size,
            )
        else:
            paged_kv_indices = None

        # Early-out for cascade attention
        if use_cascade:
            # Grab the blocks of the shared prefix from the first request.
            num_common_kv_blocks = common_prefix_len // page_size

            # Create CPU versions directly for cascade (no GPU versions needed)
            shared_qo_indptr_cpu = torch.tensor(
                [0, num_actual_tokens], dtype=torch.int32, device="cpu"
            )
            shared_kv_page_indptr_cpu = torch.tensor(
                [0, num_common_kv_blocks], dtype=torch.int32, device="cpu"
            )
            shared_kv_page_indices_cpu = block_table_tensor[0, :num_common_kv_blocks]
            shared_kv_last_page_len_cpu = torch.tensor(
                [page_size], dtype=torch.int32, device="cpu"
            )

            # Remove the blocks of the shared prefix from all requests.
            block_table_tensor = block_table_tensor[:, num_common_kv_blocks:]
            num_blocks_np -= num_common_kv_blocks

            assert paged_kv_indices is not None
            paged_kv_indptr_cpu = self.paged_kv_indptr.cpu[: 1 + num_reqs]
            paged_kv_last_page_len_cpu = self.paged_kv_last_page_len.cpu[:num_reqs]

            attn_metadata.cascade_wrapper = self._get_cascade_wrapper()
            attn_metadata.cascade_wrapper.plan(
                qo_indptr_arr=[shared_qo_indptr_cpu, qo_indptr_cpu],
                paged_kv_indptr_arr=[shared_kv_page_indptr_cpu, paged_kv_indptr_cpu],
                paged_kv_indices_arr=[shared_kv_page_indices_cpu, paged_kv_indices],
                paged_kv_last_page_len=[
                    shared_kv_last_page_len_cpu,
                    paged_kv_last_page_len_cpu,
                ],
                num_qo_heads=self.num_qo_heads,
                num_kv_heads=self.num_kv_heads,
                head_dim=self.head_dim,
                page_size=self.page_size,
                causal=True,
                sm_scale=self.sm_scale,
                window_left=self.window_left,
                logits_soft_cap=self.logits_soft_cap,
                q_data_type=self.q_data_type,
                kv_data_type=self.kv_cache_dtype,
            )
            return attn_metadata

        # Step 3: Handle prefill and decode pathways case by case
        ## PREFILL PATHWAY
        if num_prefills > 0:
            # Slices for shared prefill metadata
            prefill_start = num_decodes
            qo_indptr_prefill_cpu = (
                qo_indptr_cpu[prefill_start:] - qo_indptr_cpu[prefill_start]
            )
            assert qo_indptr_prefill_cpu.shape[0] == num_prefills + 1

            if prefill_use_trtllm:
                # Create GPU versions
                qo_indptr_prefill_gpu = (
                    qo_indptr[prefill_start:] - qo_indptr[prefill_start]
                )
                paged_kv_indptr_prefill_gpu = self.paged_kv_indptr.gpu[
                    prefill_start : num_reqs + 1
                ]
                # Compute max_q_len for prefill requests
                query_lens_prefill_cpu = (
                    qo_indptr_prefill_cpu[1:] - qo_indptr_prefill_cpu[:-1]
                )
                max_q_len_prefill = int(query_lens_prefill_cpu.max().item())
                attn_metadata.prefill = TRTLLMPrefill(
                    block_tables=block_table_tensor[prefill_start:],
                    seq_lens=seq_lens[prefill_start:],
                    cum_seq_lens_q=qo_indptr_prefill_gpu,
                    cum_seq_lens_kv=paged_kv_indptr_prefill_gpu,
                    max_q_len=max_q_len_prefill,
                    max_seq_len=max_seq_len,
                )
            else:
                prefill_wrapper = self._get_prefill_wrapper()
                # Slicing CPU buffers that are only needed for FI native prefills
                paged_kv_last_page_len_prefill_cpu = self.paged_kv_last_page_len.cpu[
                    prefill_start:num_reqs
                ]
                assert paged_kv_last_page_len_prefill_cpu.shape[0] == num_prefills
                paged_kv_indptr_prefill_cpu = self.paged_kv_indptr.cpu[
                    prefill_start : num_reqs + 1
                ]
                assert paged_kv_indptr_prefill_cpu.shape[0] == num_prefills + 1
                if self.use_dcp:
                    assert isinstance(prefill_wrapper, BatchDCPPrefillWrapper)
                    prefill_wrapper.plan(
                        qo_indptr_cpu=qo_indptr_prefill_cpu,
                        paged_kv_indptr_cpu=paged_kv_indptr_prefill_cpu,
                        paged_kv_indices=paged_kv_indices,
                        paged_kv_last_page_len_cpu=paged_kv_last_page_len_prefill_cpu,
                        page_size=self.page_size,
                        num_qo_heads=self.num_qo_heads,
                        dcp_world_size=self.dcp_world_size,
                        num_kv_heads=self.num_kv_heads,
                        head_dim=self.head_dim,
                        sm_scale=self.sm_scale,
                        window_left=self.window_left,
                        logits_soft_cap=self.logits_soft_cap,
                        q_data_type=self.q_data_type,
                        kv_cache_dtype=self.kv_cache_dtype,
                        prefill_fixed_split_size=self.prefill_fixed_split_size,
                        disable_split_kv=self.disable_split_kv,
                    )
                else:
                    assert isinstance(
                        prefill_wrapper,
                        BatchPrefillWithPagedKVCacheWrapper,
                    )
                    prefill_wrapper.plan(
                        qo_indptr=qo_indptr_prefill_cpu,
                        paged_kv_indptr=paged_kv_indptr_prefill_cpu,
                        paged_kv_indices=paged_kv_indices,
                        paged_kv_last_page_len=paged_kv_last_page_len_prefill_cpu,
                        num_qo_heads=self.num_qo_heads,
                        num_kv_heads=self.num_kv_heads,
                        head_dim_qk=self.head_dim,
                        page_size=self.page_size,
                        causal=True,
                        sm_scale=self.sm_scale,
                        window_left=self.window_left,
                        logits_soft_cap=self.logits_soft_cap,
                        q_data_type=self.q_data_type,
                        kv_data_type=self.kv_cache_dtype,
                        o_data_type=self.model_config.dtype,
                        fixed_split_size=self.prefill_fixed_split_size,
                        disable_split_kv=self.disable_split_kv,
                    )
                attn_metadata.prefill = FIPrefill(wrapper=prefill_wrapper)

        ## DECODE PATHWAY
        if num_decodes > 0:
            if decode_use_trtllm:
                assert num_decode_tokens % num_decodes == 0, (
                    "TRTLLM decode requires uniform query lengths per request."
                )
                attn_metadata.decode = TRTLLMDecode(
                    block_tables=block_table_tensor[:num_decodes],
                    seq_lens=seq_lens[:num_decodes],
                    max_seq_len=max_seq_len,
                )
            else:
                pure_decode = num_prefills == 0
                use_cudagraph = (
                    self.enable_cuda_graph
                    and pure_decode
                    and num_decode_tokens <= self._decode_cudagraph_max_bs
                )
                num_input_tokens = num_decode_tokens

                decode_wrapper = self._get_decode_wrapper(
                    num_input_tokens, use_cudagraph
                )
                # Use the persistent buffer with padding length,
                # instead of the same address but chunked version
                # in atten_metadata when using cudagraph.
                fast_plan_decode(
                    decode_wrapper,
                    indptr_cpu=self.paged_kv_indptr.cpu[: num_input_tokens + 1],
                    indices=paged_kv_indices,
                    last_page_len_cpu=self.paged_kv_last_page_len.cpu[
                        :num_input_tokens
                    ],
                    num_qo_heads=self.num_qo_heads * self.dcp_world_size,
                    num_kv_heads=self.num_kv_heads,
                    head_dim=self.head_dim,
                    page_size=self.page_size,
                    # Disable flashinfer's pos encoding and use vllm's rope.
                    pos_encoding_mode="NONE",
                    sm_scale=self.sm_scale,
                    window_left=self.window_left,
                    logits_soft_cap=self.logits_soft_cap,
                    q_data_type=self.q_data_type,
                    kv_data_type=self.kv_cache_dtype,
                    o_data_type=self.model_config.dtype,
                    fixed_split_size=self.decode_fixed_split_size,
                    disable_split_kv=self.disable_split_kv,
                )
                attn_metadata.decode = FIDecode(wrapper=decode_wrapper)
        return attn_metadata

    def use_cascade_attention(self, *args, **kwargs) -> bool:
        if self.kv_cache_spec.dtype != self.vllm_config.model_config.dtype:
            # TODO: The cascade wrapper currently does not support setting
            # kv cache dtype to something different from query dtype.
            return False
        # TODO: Cascade attention doesn't work, disable it for now
        # return use_cascade_attention(*args, **kwargs)
        return False


class FlashInferImpl(AttentionImpl):
    can_return_lse_for_decode: bool = True

    def __init__(
        self,
        num_heads: int,
        head_size: int,
        scale: float,
        num_kv_heads: int,
        alibi_slopes: list[float] | None,
        sliding_window: int | None,
        kv_cache_dtype: str,
        logits_soft_cap: float | None = None,
        attn_type: AttentionType = AttentionType.DECODER,
        kv_sharing_target_layer_name: int | None = None,
        sinks: torch.Tensor | None = None,
    ) -> None:
        self.num_heads = num_heads
        self.head_size = head_size
        self.scale = float(scale)
        self.num_kv_heads = num_kv_heads
        if alibi_slopes is not None:
            alibi_slopes = torch.tensor(alibi_slopes, dtype=torch.float32)
        self.alibi_slopes = alibi_slopes
        if sliding_window is None:
            self.sliding_window = (-1, -1)
        else:
            self.sliding_window = (sliding_window - 1, 0)
        self.window_left = (
            self.sliding_window[0] if self.sliding_window is not None else -1
        )
        self.kv_cache_dtype = kv_cache_dtype
        self.logits_soft_cap = logits_soft_cap
        self.kv_sharing_target_layer_name = kv_sharing_target_layer_name

        self.num_queries_per_kv = self.num_heads // self.num_kv_heads

        if attn_type != AttentionType.DECODER:
            raise NotImplementedError(
                "Encoder self-attention and "
                "encoder/decoder cross-attention "
                "are not implemented for "
                "FlashInferImpl"
            )

        self.sinks: torch.Tensor | None = None
        if sinks is not None:
            if sinks.shape[0] != num_heads:
                raise ValueError(
                    "Sinks must have the same number of heads as the number of "
                    f"heads in the layer. Expected {num_heads}, but got "
                    f"{sinks.shape[0]}."
                )
            self.sinks = sinks

        self.support_trtllm_attn = can_use_trtllm_attention(num_heads, num_kv_heads)
        vllm_config = get_current_vllm_config_or_none()
        self.supports_quant_query_input = (
            self.support_trtllm_attn
            and vllm_config is not None
            and not vllm_config.attention_config.disable_flashinfer_q_quantization
        )
        self.bmm1_scale: float | None = None
        self.bmm2_scale: float | None = None
        self.o_sf_scale: float | None = None

        dcp_a2a = (
            vllm_config is not None
            and vllm_config.parallel_config.decode_context_parallel_size > 1
            and vllm_config.parallel_config.dcp_comm_backend == "a2a"
        )
        if dcp_a2a:
            self.dcp_combine = partial(dcp_a2a_lse_reduce, is_lse_base_on_e=False)
        else:
            self.dcp_combine = partial(cp_lse_ag_out_rs, is_lse_base_on_e=False)

    def fused_output_quant_supported(self, quant_key: QuantKey):
        return (
            self.support_trtllm_attn
            and self.kv_cache_dtype.startswith("fp8")
            and quant_key in (kFp8StaticTensorSym, kNvfp4Dynamic)
        )

    # FlashInfer requires attention sinks to be float32
    def process_weights_after_loading(self, act_dtype: torch.dtype):
        if self.sinks is not None and self.sinks.dtype != torch.float32:
            self.sinks = self.sinks.to(torch.float32)

    def forward(
        self,
        layer: torch.nn.Module,
        query: torch.Tensor,
        key: torch.Tensor,
        value: torch.Tensor,
        kv_cache: torch.Tensor,
        attn_metadata: FlashInferMetadata,
        output: torch.Tensor | None = None,
        output_scale: torch.Tensor | None = None,
        output_block_scale: torch.Tensor | None = None,
    ) -> torch.Tensor:
        """Forward pass with FlashInfer.

        Args:
            query: shape = [num_tokens, num_heads, head_size]
            key: shape = [num_tokens, num_kv_heads, head_size]
            value: shape = [num_tokens, num_kv_heads, head_size]
            kv_cache: KV cache tensor with different possible shapes:
                - NHD: [num_blocks, 2, block_size, num_kv_heads, head_size]
                - HND: [num_blocks, 2, num_kv_heads, block_size, head_size]
            attn_metadata: Metadata for attention.
        Returns:
            shape = [num_tokens, num_heads * head_size]
        """
        assert output is not None, "Output tensor must be provided."

        if attn_metadata is None:
            # Profiling run.
            return output.fill_(0)

        # Ensure query dtype matches the expected dtype from attention metadata
        assert attn_metadata.q_data_type == query.dtype, (
            f"Query dtype mismatch: expected {attn_metadata.q_data_type}, "
            f"got {query.dtype}"
        )

        if self.bmm1_scale is None:
            self.bmm1_scale = layer._q_scale_float * layer._k_scale_float * self.scale

        if self.bmm2_scale is None:
            self.bmm2_scale = layer._v_scale_float

        prefill_use_trtllm = isinstance(attn_metadata.prefill, TRTLLMPrefill)
        decode_use_trtllm = isinstance(attn_metadata.decode, TRTLLMDecode)

        # The attn+quant fusion happens when output_scale is provided.
        if output_scale is None:
            assert output_block_scale is None, (
                "output_block_scale is not supported when fusion has not happened"
            )
        else:
            assert attn_metadata.q_data_type == FP8_DTYPE, (
                "Query must be FP8 when attn+quant fusion happened."
            )
            assert (attn_metadata.num_prefills == 0 or prefill_use_trtllm) and (
                attn_metadata.num_decodes == 0 or decode_use_trtllm
            ), "Must use TRT-LLM attn"

            if output.dtype == FP8_DTYPE:
                assert output_block_scale is None, (
                    "output_block_scale should not be provided for fp8 output"
                )
            elif output.dtype == FP4_DTYPE:
                assert output_block_scale is not None, (
                    "output_block_scale is required for nvfp4 output"
                )
            else:
                raise ValueError(f"Unsupported output dtype: {output.dtype}")

            # TRTLLM attn kernel requires to scale to pass as a host scalar,
            # store the o scale as a host scalar in warmup run with cuda graph
            # not enabled
            if layer._o_scale_float is None:
                layer._o_scale_float = output_scale.cpu().item()
                if output.dtype == FP8_DTYPE:
                    self.bmm2_scale = self.bmm2_scale / layer._o_scale_float
                elif output.dtype == FP4_DTYPE:
                    self.o_sf_scale = layer._o_scale_float

        # IMPORTANT!
        # NOTE(woosuk): With piece-wise CUDA graphs, this method is executed in
        # eager-mode PyTorch. Thus, we need to be careful about any CPU overhead
        # in this method. For example, `view` and `slice` (or `[:n]`) operations
        # are surprisingly slow even in the case they do not invoke any GPU ops.
        # Minimize the PyTorch ops in this method as much as possible.
        # Whenever making a change in this method, please benchmark the
        # performance to make sure it does not introduce any overhead.

        num_actual_tokens = attn_metadata.num_actual_tokens

        # The FlashInfer api requires data to be in fp8_e4m3 or fp8_e5m2
        # to process the cache when the kv_cache_dtype is fp8
        if self.kv_sharing_target_layer_name is None and self.kv_cache_dtype.startswith(
            "fp8"
        ):
            torch_dtype = FlashInferBackend.get_fp8_dtype_for_flashinfer(
                self.kv_cache_dtype
            )
            kv_cache = kv_cache.view(torch_dtype)

        # Inputs and outputs may be padded for CUDA graphs
        query = query[:num_actual_tokens]
        key = key[:num_actual_tokens]
        value = value[:num_actual_tokens]
        output_padded = output
        output = output[:num_actual_tokens]

        if attn_metadata.use_cascade:
            # Cascade attention (rare case).
            assert attn_metadata.cascade_wrapper is not None
            output.copy_(attn_metadata.cascade_wrapper.run(query, kv_cache))
            return output

        # When using spec decoding, num_decodes can be < num_decode_tokens
        # because some decode requests may have more than one query token.
        num_decode_tokens = attn_metadata.num_decode_tokens
        num_prefill_tokens = attn_metadata.num_prefill_tokens

        stride_order = FlashInferBackend.get_kv_cache_stride_order()
        kv_cache_permute = kv_cache.permute(*stride_order)

        use_dcp = self.dcp_world_size > 1

        # Regular attention (common case).
        # Decodes are at the front and prefills are at the back.
        if num_prefill_tokens > 0:
            prefill_query = query[num_decode_tokens:]
            assert prefill_query.shape[0] == num_prefill_tokens

            if not prefill_use_trtllm:
                assert isinstance(attn_metadata.prefill, FIPrefill)
                prefill_wrapper = attn_metadata.prefill.wrapper
                assert prefill_wrapper is not None
                if use_dcp:
                    assert isinstance(prefill_wrapper, BatchDCPPrefillWrapper)
                    assert prefill_wrapper._context._window_left == self.window_left
                    assert prefill_wrapper._context._logits_soft_cap == (
                        self.logits_soft_cap or 0.0
                    )
                    assert prefill_wrapper._context._sm_scale == self.scale
                    assert not prefill_wrapper._context._causal
                    assert prefill_wrapper._new_tokens._window_left == self.window_left
                    assert prefill_wrapper._new_tokens._logits_soft_cap == (
                        self.logits_soft_cap or 0.0
                    )
                    assert prefill_wrapper._new_tokens._sm_scale == self.scale
                    assert prefill_wrapper._new_tokens._causal

                    prefill_wrapper.run(
                        layer,
                        prefill_query,
                        kv_cache_permute,
                        key[num_decode_tokens:],
                        value[num_decode_tokens:],
                        out=output[num_decode_tokens:],
                    )
                else:
                    assert isinstance(
                        prefill_wrapper, BatchPrefillWithPagedKVCacheWrapper
                    )
                    assert prefill_wrapper._window_left == self.window_left
                    assert prefill_wrapper._logits_soft_cap == (
                        self.logits_soft_cap or 0.0
                    )
                    assert prefill_wrapper._sm_scale == self.scale
                    assert prefill_wrapper._causal
                    prefill_wrapper.run(
                        prefill_query,
                        kv_cache_permute,
                        k_scale=layer._k_scale_float,
                        v_scale=layer._v_scale_float,
                        out=output[num_decode_tokens:],
                    )
            else:
                assert isinstance(attn_metadata.prefill, TRTLLMPrefill)
                # prefill_query may be non-contiguous or have degenerate strides
                # First ensure memory contiguity, then fix degenerate strides
                # with reshape. contiguous() alone doesn't fix degenerate
                # strides when a dimension has size 1.
                prefill_query = prefill_query.contiguous().reshape(prefill_query.shape)
                workspace_buffer = _get_trtllm_gen_workspace_buffer()
                block_tables_prefill = attn_metadata.prefill.block_tables
                seq_lens_prefill = attn_metadata.prefill.seq_lens

                # This path needs to be enabled with VLLM_KV_CACHE_LAYOUT = HND
                assert get_kv_cache_layout() == "HND"
                assert is_strictly_contiguous(prefill_query)
                assert is_strictly_contiguous(kv_cache_permute)
                assert is_strictly_contiguous(workspace_buffer)
                assert is_strictly_contiguous(block_tables_prefill)
                assert is_strictly_contiguous(seq_lens_prefill)

                if output.dtype == FP4_DTYPE:
                    assert self.o_sf_scale is not None
                    out = FP4Tensor(
                        data=output[num_decode_tokens:],
                        scale=output_block_scale,
                        scale_start_index=num_decode_tokens,
                        original_shape=prefill_query.shape,
                    )
                else:
                    assert self.o_sf_scale is None
                    out = output[num_decode_tokens:]

                if (
                    attn_metadata.q_data_type != FP8_DTYPE
                    and self.kv_cache_dtype.startswith("fp8")
                ):
                    # TRTLLM prefill attention does not support BF16 Q
                    # and fp8 kv cache. So to enable prefill attention
                    # with fp8 kv cache, we can construct a mock block
                    # and mock kv cache with BF16 KV involved in the prefill
                    mock_kv_cache, mock_block_table = trtllm_prefill_attn_kvfp8_dequant(
                        kv_cache_permute,
                        block_tables_prefill,
                        layer._k_scale,
                        layer._v_scale,
                        attn_metadata.q_data_type,
                    )
                else:
                    mock_kv_cache = kv_cache_permute
                    mock_block_table = block_tables_prefill

                trtllm_batch_context_with_kv_cache(
                    query=prefill_query,
                    kv_cache=mock_kv_cache,
                    workspace_buffer=workspace_buffer,
                    block_tables=mock_block_table,
                    seq_lens=seq_lens_prefill,
                    max_q_len=attn_metadata.prefill.max_q_len,
                    max_kv_len=attn_metadata.prefill.max_seq_len,
                    bmm1_scale=self.bmm1_scale,
                    bmm2_scale=self.bmm2_scale,
                    batch_size=attn_metadata.num_prefills,
                    cum_seq_lens_q=attn_metadata.prefill.cum_seq_lens_q,
                    cum_seq_lens_kv=attn_metadata.prefill.cum_seq_lens_kv,
                    window_left=self.window_left,
                    sinks=self.sinks,
                    o_sf_scale=self.o_sf_scale,
                    out=out,
                )

        if num_decode_tokens > 0:
            decode_query = query[:num_decode_tokens]
            assert decode_query.shape[0] == num_decode_tokens

            if not decode_use_trtllm:
                assert isinstance(attn_metadata.decode, FIDecode)
                decode_wrapper = attn_metadata.decode.wrapper
                assert decode_wrapper is not None
                assert decode_wrapper._window_left == self.window_left
                assert decode_wrapper._logits_soft_cap == (self.logits_soft_cap or 0.0)
                assert decode_wrapper._sm_scale == self.scale

                if use_dcp:
                    decode_query = get_dcp_group().all_gather(
                        decode_query.contiguous(), dim=-2
                    )
                    output_tmp = torch.empty_like(decode_query)
                    lse = torch.empty(
                        (decode_query.size(0), decode_query.size(1)),
                        dtype=torch.float32,
                        device=decode_query.device,
                    )
                    decode_wrapper.run(
                        decode_query,
                        kv_cache_permute,
                        k_scale=layer._k_scale_float,
                        v_scale=layer._v_scale_float,
                        out=output_tmp,
                        lse=lse,
                        return_lse=True,
                    )
                    output[:num_decode_tokens] = self.dcp_combine(
                        output_tmp,
                        lse,
                        get_dcp_group(),
                    )
                else:
                    decode_wrapper.run(
                        decode_query,
                        kv_cache_permute,
                        k_scale=layer._k_scale_float,
                        v_scale=layer._v_scale_float,
                        out=output[:num_decode_tokens],
                    )
            else:
                # decode_query may be non-contiguous or have degenerate strides
                assert isinstance(attn_metadata.decode, TRTLLMDecode)
                # First ensure memory contiguity, then fix degenerate strides
                # with reshape. contiguous() alone doesn't fix degenerate
                # strides when a dimension has size 1.
                decode_query = decode_query.contiguous().reshape(decode_query.shape)
                workspace_buffer = _get_trtllm_gen_workspace_buffer()
                block_tables_decode = attn_metadata.decode.block_tables
                seq_lens_decode = attn_metadata.decode.seq_lens

                # This path needs to be enabled with VLLM_KV_CACHE_LAYOUT = HND
                assert get_kv_cache_layout() == "HND"
                assert is_strictly_contiguous(decode_query)
                assert is_strictly_contiguous(kv_cache_permute)
                assert is_strictly_contiguous(workspace_buffer)
                assert is_strictly_contiguous(block_tables_decode)
                assert is_strictly_contiguous(seq_lens_decode)

                if output.dtype == FP4_DTYPE:
                    assert self.o_sf_scale is not None
                    out = FP4Tensor(
                        data=output[:num_decode_tokens],
                        scale=output_block_scale,
                        scale_start_index=0,
                        original_shape=decode_query.shape,
                    )
                else:
                    assert self.o_sf_scale is None
                    out = output[:num_decode_tokens]

                if num_decode_tokens % attn_metadata.num_decodes != 0:
                    # This gets triggered when the dummy_run forces
                    # attention to be initialized with q_len = 0
                    q_len_per_req = 1
                else:
                    q_len_per_req = num_decode_tokens // attn_metadata.num_decodes

                trtllm_batch_decode_with_kv_cache(
                    query=decode_query,
                    kv_cache=kv_cache_permute,
                    workspace_buffer=workspace_buffer,
                    block_tables=block_tables_decode,
                    seq_lens=seq_lens_decode,
                    max_seq_len=attn_metadata.decode.max_seq_len,
                    bmm1_scale=self.bmm1_scale,
                    bmm2_scale=self.bmm2_scale,
                    window_left=self.window_left,
                    sinks=self.sinks,
                    o_sf_scale=self.o_sf_scale,
                    out=out,
                    q_len_per_req=q_len_per_req,
                )
        return output_padded

    def do_kv_cache_update(
        self,
        layer: torch.nn.Module,
        key: torch.Tensor,
        value: torch.Tensor,
        kv_cache: torch.Tensor,
        slot_mapping: torch.Tensor,
    ) -> None:
        if self.kv_sharing_target_layer_name is None:
            # Reshape the input keys and values and store them in the cache.
            # Skip this if sharing KV cache with an earlier attention layer.
            # NOTE(woosuk): Here, key and value are padded while slot_mapping is
            # not padded. However, we don't need to do key[:num_actual_tokens]
            # and value[:num_actual_tokens] because the reshape_and_cache_flash
            # op uses the slot_mapping's shape to determine the number of
            # actual tokens.
            torch.ops._C_cache_ops.reshape_and_cache_flash(
                key,
                value,
                kv_cache[:, 0],
                kv_cache[:, 1],
                slot_mapping,
                self.kv_cache_dtype,
                layer._k_scale,
                layer._v_scale,
            )


def fast_plan_decode(
    self,  # decode wrapper
    indptr_cpu: torch.Tensor,
    indices: torch.Tensor,
    last_page_len_cpu: torch.Tensor,
    num_qo_heads: int,
    num_kv_heads: int,
    head_dim: int,
    page_size: int,
    pos_encoding_mode: str = "NONE",
    window_left: int = -1,
    logits_soft_cap: float | None = None,
    q_data_type: str | torch.dtype | None = "float16",
    kv_data_type: str | torch.dtype | None = None,
    o_data_type: str | torch.dtype | None = None,
    data_type: str | torch.dtype | None = None,
    sm_scale: float | None = None,
    rope_scale: float | None = None,
    rope_theta: float | None = None,
    non_blocking: bool = True,
    fixed_split_size: int = -1,
    disable_split_kv: bool = False,
) -> None:
    """
    A faster version of BatchDecodeWithPagedKVCacheWrapper::plan used for
    cudagraph capture/replay, while the no cudagraph version turns back
    to the original plan.
    using original plan after passing host-side buffers:
    - only host-to-device copy of indptr and last_page_len buffers
    Modifications for cudagraph:
    - only host-to-device copy of indptr and last_page_len buffers.
    - avoid device-to-device copy of indices buffer.

    Part of the code get inspiration from the original plan from FlashInfer repo
    and the implementation of fast_decode_plan for FlashInfer in SGlang repo.
    """
    # Warm up with the original plan if it is first call, and always run the
    # original plan if we run for dynamic shape. For fixed shape (cudagraph),
    # this warm up is to generate the _cached_module for the decode wrapper.
    if not self.is_cuda_graph_enabled or getattr(self, "vllm_first_call", True):
        self.plan(
            indptr=indptr_cpu,
            indices=indices,
            last_page_len=last_page_len_cpu,
            num_qo_heads=num_qo_heads,
            num_kv_heads=num_kv_heads,
            head_dim=head_dim,
            page_size=page_size,
            pos_encoding_mode=pos_encoding_mode,
            window_left=window_left,
            logits_soft_cap=logits_soft_cap,
            q_data_type=q_data_type,
            kv_data_type=kv_data_type,
            o_data_type=o_data_type,
            data_type=data_type,
            sm_scale=sm_scale,
            rope_scale=rope_scale,
            rope_theta=rope_theta,
            non_blocking=non_blocking,
            block_tables=None,
            seq_lens=None,
            fixed_split_size=fixed_split_size,
            disable_split_kv=disable_split_kv,
        )
        self.vllm_first_call = False
        return

    assert self.is_cuda_graph_enabled, "Should be cudagraph only here"

    fast_decode_plan(
        self,
        indptr=indptr_cpu,
        indices=indices,
        last_page_len=last_page_len_cpu,
        num_qo_heads=num_qo_heads,
        num_kv_heads=num_kv_heads,
        head_dim=head_dim,
        page_size=page_size,
        pos_encoding_mode=pos_encoding_mode,
        window_left=window_left,
        logits_soft_cap=logits_soft_cap,
        q_data_type=q_data_type,
        kv_data_type=kv_data_type,
        data_type=data_type,
        sm_scale=sm_scale,
        rope_scale=rope_scale,
        rope_theta=rope_theta,
        non_blocking=non_blocking,
        fixed_split_size=fixed_split_size,
        disable_split_kv=disable_split_kv,
    )


@triton.jit
def _copy_page_indices_kernel(
    page_indices,
    block_table,
    block_table_stride,
    cu_num_blocks,
    BLOCK_SIZE: tl.constexpr,
):
    req_idx = tl.program_id(0)
    row_ptr = block_table + req_idx * block_table_stride
    start_idx = tl.load(cu_num_blocks + req_idx)
    end_idx = tl.load(cu_num_blocks + req_idx + 1)
    num_blocks = end_idx - start_idx

    offset = tl.arange(0, BLOCK_SIZE)
    for i in tl.range(0, num_blocks, BLOCK_SIZE):
        block_ids = tl.load(row_ptr + i + offset, mask=i + offset < num_blocks)
        tl.store(
            page_indices + start_idx + i + offset,
            block_ids,
            mask=i + offset < num_blocks,
        )