[Attention] Move MLA `forward` from backend to layer (#33284)

Signed-off-by: Matthew Bonanni <mbonanni@redhat.com>

tests/v1/attention/test_mla_backends.py

@@ -274,11 +274,157 @@ class MockAttentionLayer:
         raise NotImplementedError
 
 
+class MockSparseMLAAttentionLayer:
+    """A mock sparse MLA attention layer for testing.
+
+    Sparse MLA implementations only support forward_mqa (decode-style attention)
+    for all tokens, so this class only implements that path.
+
+    Unlike regular MLA impls, sparse MLA impls don't have W_UK_T and W_UV
+    attributes. These transformations are done by the layer (MLAAttention),
+    not the impl. This mock layer accepts these weight matrices directly.
+    """
+
+    def __init__(
+        self,
+        impl,
+        num_heads: int,
+        qk_nope_head_dim: int,
+        qk_rope_head_dim: int,
+        v_head_dim: int,
+        kv_lora_rank: int,
+        device: torch.device,
+        W_UK: torch.Tensor,
+        W_UV: torch.Tensor,
+    ):
+        self.impl = impl
+        self.num_heads = num_heads
+        self.qk_nope_head_dim = qk_nope_head_dim
+        self.qk_rope_head_dim = qk_rope_head_dim
+        self.v_head_dim = v_head_dim
+        self.kv_lora_rank = kv_lora_rank
+
+        # Compute weight matrices in the format expected by forward_impl
+        # W_UK shape: (L, N, P) -> W_UK_T shape: (N, P, L)
+        self.W_UK_T = W_UK.permute(1, 2, 0)
+        # W_UV shape: (L, N, V) -> (N, L, V)
+        self.W_UV = W_UV.transpose(0, 1)
+
+        # Scale attributes needed by attention backends
+        self._q_scale = torch.tensor(1.0, device=device)
+        self._k_scale = torch.tensor(1.0, device=device)
+        self._v_scale = torch.tensor(1.0, device=device)
+        self._prob_scale = torch.tensor(1.0, device=device)
+        self._q_scale_float = 1.0
+        self._k_scale_float = 1.0
+        self._v_scale_float = 1.0
+
+    def forward_impl(
+        self,
+        q: torch.Tensor,
+        kv_c: torch.Tensor,
+        k_pe: torch.Tensor,
+        kv_cache: torch.Tensor,
+        attn_metadata,
+        output: torch.Tensor,
+    ) -> torch.Tensor:
+        """Forward for sparse MLA - uses forward_mqa for all tokens."""
+        # Write to KV cache
+        kv_cache_dtype = getattr(self.impl, "kv_cache_dtype", "auto")
+        if kv_cache.numel() > 0:
+            ops.concat_and_cache_mla(
+                kv_c,
+                k_pe.squeeze(1),
+                kv_cache,
+                attn_metadata.slot_mapping.flatten(),
+                kv_cache_dtype=kv_cache_dtype,
+                scale=self._k_scale,
+            )
+
+        num_tokens = q.shape[0]
+
+        # Sparse MLA uses forward_mqa for all tokens
+        # Split q into nope and pe parts
+        mqa_q_nope, mqa_q_pe = q.split(
+            [self.qk_nope_head_dim, self.qk_rope_head_dim], dim=-1
+        )
+
+        # Convert from (B, N, P) to (N, B, P)
+        mqa_q_nope = mqa_q_nope.transpose(0, 1)
+
+        # Multiply (N, B, P) x (N, P, L) -> (N, B, L)
+        mqa_ql_nope = torch.bmm(mqa_q_nope, self.W_UK_T)
+
+        # Convert from (N, B, L) to (B, N, L)
+        mqa_ql_nope = mqa_ql_nope.transpose(0, 1)
+
+        # Pass as tuple to forward_mqa
+        mqa_q = (mqa_ql_nope, mqa_q_pe)
+
+        attn_out, _ = self.impl.forward_mqa(mqa_q, kv_cache, attn_metadata, self)
+
+        # v_up projection: multiply by W_UV
+        # attn_out shape: (B, N, L) where L = kv_lora_rank
+        # W_UV shape: (N, L, V)
+        # output shape: (B, N, V) -> flatten to (B, N*V)
+        decode_output = torch.bmm(attn_out.transpose(0, 1), self.W_UV).transpose(0, 1)
+        output[:num_tokens] = decode_output.reshape(
+            num_tokens, self.num_heads * self.v_head_dim
+        )
+
+        return output
+
+
 class MockMLAAttentionLayer(AttentionLayerBase):
-    """A mock MLA attention layer for populating static_forward_context."""
+    """A mock MLA attention layer for testing.
+
+    This replicates the forward_impl logic from MLAAttention to allow
+    testing MLA backends without the full layer infrastructure.
+
+    The W_UK_T and W_UV weight matrices are created on the layer (like in
+    MLAAttention.process_weights_after_loading), not on the impl.
+    """
 
-    def __init__(self, impl):
+    def __init__(
+        self,
+        impl,
+        num_heads: int,
+        qk_nope_head_dim: int,
+        qk_rope_head_dim: int,
+        v_head_dim: int,
+        kv_lora_rank: int,
+        device: torch.device,
+        kv_b_proj,
+    ):
         self.impl = impl
+        self.num_heads = num_heads
+        self.qk_nope_head_dim = qk_nope_head_dim
+        self.qk_rope_head_dim = qk_rope_head_dim
+        self.v_head_dim = v_head_dim
+        self.kv_lora_rank = kv_lora_rank
+
+        # Compute weight matrices from kv_b_proj (like MLAAttention does)
+        # This replicates MLAAttention.process_weights_after_loading logic
+        kv_b_proj_weight = kv_b_proj.weight.T
+        kv_b_proj_weight = kv_b_proj_weight.view(
+            kv_lora_rank,
+            num_heads,
+            qk_nope_head_dim + v_head_dim,
+        )
+        W_UK, W_UV = kv_b_proj_weight.split([qk_nope_head_dim, v_head_dim], dim=-1)
+        # Convert from (L, N, V) to (N, L, V)
+        self.W_UV = W_UV.transpose(0, 1)
+        # Convert from (L, N, P) to (N, P, L)
+        self.W_UK_T = W_UK.permute(1, 2, 0)
+
+        # Scale attributes needed by attention backends
+        self._q_scale = torch.tensor(1.0, device=device)
+        self._k_scale = torch.tensor(1.0, device=device)
+        self._v_scale = torch.tensor(1.0, device=device)
+        self._prob_scale = torch.tensor(1.0, device=device)
+        self._q_scale_float = 1.0
+        self._k_scale_float = 1.0
+        self._v_scale_float = 1.0
 
     def get_attn_backend(self):
         raise NotImplementedError
@@ -286,6 +432,83 @@ class MockMLAAttentionLayer(AttentionLayerBase):
     def get_kv_cache_spec(self, vllm_config):
         raise NotImplementedError
 
+    def forward_impl(
+        self,
+        q: torch.Tensor,
+        kv_c: torch.Tensor,
+        k_pe: torch.Tensor,
+        kv_cache: torch.Tensor,
+        attn_metadata,
+        output: torch.Tensor,
+    ) -> torch.Tensor:
+        """Replicates MLAAttention.forward_impl logic for testing."""
+        # Write to KV cache
+        if kv_cache.numel() > 0:
+            ops.concat_and_cache_mla(
+                kv_c,
+                k_pe.squeeze(1),
+                kv_cache,
+                attn_metadata.slot_mapping.flatten(),
+                kv_cache_dtype="auto",
+                scale=self._k_scale,
+            )
+
+        # Determine decode vs prefill split
+        num_decode_tokens = attn_metadata.num_decode_tokens or 0
+        has_decode = (attn_metadata.num_decodes or 0) > 0
+        has_prefill = (attn_metadata.num_prefills or 0) > 0
+
+        # Run prefill with forward_mha
+        if has_prefill:
+            prefill_q = q[num_decode_tokens:]
+            prefill_k_pe = k_pe[num_decode_tokens:]
+            prefill_k_c = kv_c[num_decode_tokens:]
+            self.impl.forward_mha(
+                prefill_q,
+                prefill_k_c,
+                prefill_k_pe,
+                kv_cache,
+                attn_metadata,
+                self._k_scale,
+                output=output[num_decode_tokens:],
+            )
+
+        # Run decode with forward_mqa
+        if has_decode:
+            decode_q = q[:num_decode_tokens]
+
+            # Split q into nope and pe parts
+            mqa_q_nope, mqa_q_pe = decode_q.split(
+                [self.qk_nope_head_dim, self.qk_rope_head_dim], dim=-1
+            )
+
+            # Convert from (B, N, P) to (N, B, P)
+            mqa_q_nope = mqa_q_nope.transpose(0, 1)
+
+            # Multiply (N, B, P) x (N, P, L) -> (N, B, L)
+            mqa_ql_nope = torch.bmm(mqa_q_nope, self.W_UK_T)
+
+            # Convert from (N, B, L) to (B, N, L)
+            mqa_ql_nope = mqa_ql_nope.transpose(0, 1)
+
+            # Pass as tuple to forward_mqa
+            mqa_q = (mqa_ql_nope, mqa_q_pe)
+
+            attn_out, _ = self.impl.forward_mqa(mqa_q, kv_cache, attn_metadata, self)
+
+            # v_up projection: multiply by W_UV
+            # attn_out shape: (B, N, L) where L = kv_lora_rank
+            # W_UV shape: (N, L, V)
+            # output shape: (B, N, V) -> flatten to (B, N*V)
+            decode_output = torch.bmm(attn_out.transpose(0, 1), self.W_UV).transpose(
+                0, 1
+            )
+            output[:num_decode_tokens] = decode_output.reshape(
+                num_decode_tokens, self.num_heads * self.v_head_dim
+            )
+
+        return output
+
 
 def run_attention_backend(
     backend: AttentionBackendEnum,
@@ -340,14 +563,31 @@ def run_attention_backend(
             kv_b_proj=mock_kv_b_proj,
         )
 
-        # Process weights to create W_UK_T and W_UV attributes needed by MLA
+        # Process weights on the impl
         act_dtype = _convert_dtype_to_torch(vllm_config.model_config.dtype)
         impl.process_weights_after_loading(act_dtype)
 
+        # Initialize DCP attributes (normally set by MLAAttention.forward
+        # before calling forward_mha, see mla_attention.py:511-512)
+        if impl.dcp_world_size == -1:
+            impl.dcp_world_size = 1
+
+        # Create mock MLA layer
+        mock_layer = MockMLAAttentionLayer(
+            impl=impl,
+            num_heads=num_heads,
+            qk_nope_head_dim=qk_nope_head_dim,
+            qk_rope_head_dim=qk_rope_head_dim,
+            v_head_dim=v_head_dim,
+            kv_lora_rank=kv_lora_rank,
+            device=device,
+            kv_b_proj=mock_kv_b_proj,
+        )
+
         # Populate static_forward_context with mock attention layers
         for layer_name in layer_names:
             vllm_config.compilation_config.static_forward_context[layer_name] = (
-                MockMLAAttentionLayer(impl)
+                mock_layer
             )
 
         # Build metadata
@@ -357,18 +597,15 @@ def run_attention_backend(
             common_attn_metadata=common_attn_metadata,
         )
 
-        # Create mock layer and output buffer
-        mock_layer = MockAttentionLayer(device)
+        # Create output buffer
         num_tokens = query.shape[0]
         output = torch.empty(
             num_tokens, num_heads * v_head_dim, dtype=query.dtype, device=query.device
         )
 
         # Run forward pass
-        # NOTE: The query, key, and value are already shaped correctly
-        # in the calling test function.
-        output = impl.forward(
-            mock_layer, query, kv_c, k_pe, kv_cache, attn_metadata, output=output
+        output = mock_layer.forward_impl(
+            query, kv_c, k_pe, kv_cache, attn_metadata, output
         )
 
         return output

tests/v1/attention/test_sparse_mla_backends.py

@@ -12,7 +12,7 @@ import torch
 from tests.v1.attention.test_mla_backends import (
     BATCH_SPECS,
     BatchSpec,
-    MockAttentionLayer,
+    MockSparseMLAAttentionLayer,
     create_and_prepopulate_kv_cache,
 )
 from tests.v1.attention.utils import (
@@ -408,20 +408,31 @@ def test_sparse_backend_decode_correctness(
 
         impl.process_weights_after_loading(dtype)
 
-    layer = MockAttentionLayer(device)
+        # Create mock sparse MLA layer with weight matrices
+        mock_layer = MockSparseMLAAttentionLayer(
+            impl=impl,
+            num_heads=num_heads,
+            qk_nope_head_dim=qk_nope_head_dim,
+            qk_rope_head_dim=qk_rope_head_dim,
+            v_head_dim=v_head_dim,
+            kv_lora_rank=kv_lora_rank,
+            device=device,
+            W_UK=W_UK,
+            W_UV=W_UV,
+        )
+
     out_buffer = torch.empty(
         metadata.num_actual_tokens, num_heads * v_head_dim, dtype=dtype, device=device
     )
 
     with torch.inference_mode():
-        backend_output = impl.forward(
-            layer,
+        backend_output = mock_layer.forward_impl(
             query_vllm,
             kv_c_vllm,
             k_pe_vllm,
             kv_cache,
             metadata,
-            output=out_buffer,
+            out_buffer,
         )
 
     assert backend_output.shape == sdpa_reference.shape

vllm/model_executor/layers/attention/attention.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Any
 
 import torch
 import torch.nn as nn
@@ -562,7 +562,7 @@ direct_register_custom_op(
 
 def get_attention_context(
     layer_name: str,
-) -> tuple[dict | object | None, "Attention | MLAAttention", torch.Tensor]:
+) -> tuple[Any, "Attention | MLAAttention", torch.Tensor]:
     """Extract attention context for a given layer.
 
     This helper function extracts the attention metadata, attention layer

vllm/v1/attention/backend.py

@@ -67,7 +67,7 @@ class AttentionBackend(ABC):
 
     @staticmethod
     @abstractmethod
-    def get_impl_cls() -> type["AttentionImpl"]:
+    def get_impl_cls() -> type["AttentionImplBase"]:
         raise NotImplementedError
 
     @staticmethod
@@ -594,7 +594,14 @@ class AttentionLayer(Protocol):
     ) -> torch.Tensor: ...
 
 
-class AttentionImpl(ABC, Generic[T]):
+class AttentionImplBase(ABC, Generic[T]):
+    """Base class for attention implementations.
+
+    Contains common attributes and initialization logic shared by both
+    standard AttentionImpl and MLAAttentionImpl. Does not define a forward
+    method - subclasses define their own forward interfaces.
+    """
+
     # Required attributes that all impls should have
     num_heads: int
     head_size: int
@@ -662,6 +669,13 @@ class AttentionImpl(ABC, Generic[T]):
         )
         return self
 
+    def process_weights_after_loading(self, act_dtype: torch.dtype):
+        pass
+
+
+class AttentionImpl(AttentionImplBase[T], Generic[T]):
+    """Standard attention implementation with forward method."""
+
     @abstractmethod
     def __init__(
         self,
@@ -704,11 +718,10 @@ class AttentionImpl(ABC, Generic[T]):
         """
         return False
 
-    def process_weights_after_loading(self, act_dtype: torch.dtype):
-        pass
 
+class MLAAttentionImpl(AttentionImplBase[T], Generic[T]):
+    """MLA attention implementation with forward_mqa and forward_mha methods."""
 
-class MLAAttentionImpl(AttentionImpl[T], Generic[T]):
     @abstractmethod
     def __init__(
         self,
@@ -731,22 +744,78 @@ class MLAAttentionImpl(AttentionImpl[T], Generic[T]):
         v_head_dim: int,
         kv_b_proj: "ColumnParallelLinear",
         indexer: object | None = None,
+        q_pad_num_heads: int | None = None,
     ) -> None:
         raise NotImplementedError
 
     @abstractmethod
-    def forward(
+    def forward_mha(
         self,
-        layer: AttentionLayer,
-        hidden_states_or_cq: torch.Tensor,
+        q: torch.Tensor,
         kv_c_normed: torch.Tensor,
         k_pe: torch.Tensor,
-        kv_cache: torch.Tensor,
+        kv_c_and_k_pe_cache: torch.Tensor,
         attn_metadata: T,
-        output: torch.Tensor | None = None,
-        output_scale: torch.Tensor | None = None,
-        output_block_scale: torch.Tensor | None = None,
-    ) -> torch.Tensor:
+        k_scale: torch.Tensor,
+        output: torch.Tensor,
+    ) -> None:
+        """MHA-style prefill forward pass."""
+        raise NotImplementedError
+
+    @abstractmethod
+    def forward_mqa(
+        self,
+        q: torch.Tensor | tuple[torch.Tensor, torch.Tensor],
+        kv_c_and_k_pe_cache: torch.Tensor,
+        attn_metadata: T,
+        layer: AttentionLayer,
+    ) -> tuple[torch.Tensor, torch.Tensor | None]:
+        """MQA-style decode forward pass."""
+        raise NotImplementedError
+
+
+class SparseMLAAttentionImpl(AttentionImplBase[T], Generic[T]):
+    """Sparse MLA attention implementation with only forward_mqa method.
+
+    Sparse MLA implementations only support decode (MQA-style) attention.
+    They do not support prefill (MHA-style) attention.
+    """
+
+    @abstractmethod
+    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,
+        attn_type: str,
+        kv_sharing_target_layer_name: str | None,
+        # MLA Specific Arguments
+        q_lora_rank: int | None,
+        kv_lora_rank: int,
+        qk_nope_head_dim: int,
+        qk_rope_head_dim: int,
+        qk_head_dim: int,
+        v_head_dim: int,
+        kv_b_proj: "ColumnParallelLinear",
+        indexer: object | None = None,
+        q_pad_num_heads: int | None = None,
+    ) -> None:
+        raise NotImplementedError
+
+    @abstractmethod
+    def forward_mqa(
+        self,
+        q: torch.Tensor | tuple[torch.Tensor, torch.Tensor],
+        kv_c_and_k_pe_cache: torch.Tensor,
+        attn_metadata: T,
+        layer: AttentionLayer,
+    ) -> tuple[torch.Tensor, torch.Tensor | None]:
+        """MQA-style decode forward pass."""
         raise NotImplementedError
 
 

vllm/v1/attention/backends/mla/cutlass_mla.py

@@ -244,7 +244,7 @@ class CutlassMLAImpl(MLACommonImpl[MLACommonMetadata]):
 
         return out, lse
 
-    def _forward_decode(
+    def forward_mqa(
         self,
         q: torch.Tensor | tuple[torch.Tensor, torch.Tensor],
         kv_c_and_k_pe_cache: torch.Tensor,

vllm/v1/attention/backends/mla/flashattn_mla.py

@@ -293,7 +293,7 @@ class FlashAttnMLAImpl(MLACommonImpl[FlashAttnMLAMetadata]):
                 "FlashAttnMLA V1 with FP8 KV cache not yet supported"
             )
 
-    def _forward_decode(
+    def forward_mqa(
         self,
         q: torch.Tensor | tuple[torch.Tensor, torch.Tensor],
         kv_c_and_k_pe_cache: torch.Tensor,

vllm/v1/attention/backends/mla/flashinfer_mla.py

@@ -150,7 +150,7 @@ class FlashInferMLAImpl(MLACommonImpl[MLACommonMetadata]):
         self.bmm1_scale: float | None = None
         self.bmm2_scale: float | None = None
 
-    def _forward_decode(
+    def forward_mqa(
         self,
         q: torch.Tensor | tuple[torch.Tensor, torch.Tensor],
         kv_c_and_k_pe_cache: torch.Tensor,

vllm/v1/attention/backends/mla/flashmla.py

@@ -234,7 +234,7 @@ class FlashMLAImpl(MLACommonImpl[FlashMLAMetadata]):
                 "FlashMLAImpl"
             )
 
-    def _forward_decode(
+    def forward_mqa(
         self,
         q: torch.Tensor | tuple[torch.Tensor, torch.Tensor],
         kv_c_and_k_pe_cache: torch.Tensor,

vllm/v1/attention/backends/mla/flashmla_sparse.py

@@ -11,7 +11,6 @@ from vllm.config import VllmConfig, get_current_vllm_config
 from vllm.config.cache import CacheDType
 from vllm.logger import init_logger
 from vllm.model_executor.layers.attention.mla_attention import (
-    MLACommonBaseImpl,
     get_mla_dims,
 )
 from vllm.platforms import current_platform
@@ -25,6 +24,7 @@ from vllm.v1.attention.backend import (
     AttentionMetadataBuilder,
     CommonAttentionMetadata,
     MultipleOf,
+    SparseMLAAttentionImpl,
 )
 from vllm.v1.attention.backends.utils import (
     reshape_attn_output_for_spec_decode,
@@ -686,7 +686,7 @@ class FlashMLASparseMetadataBuilder(AttentionMetadataBuilder[FlashMLASparseMetad
         return metadata
 
 
-class FlashMLASparseImpl(MLACommonBaseImpl[FlashMLASparseMetadata]):
+class FlashMLASparseImpl(SparseMLAAttentionImpl[FlashMLASparseMetadata]):
     @staticmethod
     def _compute_fp8_decode_padded_heads(num_heads: int) -> int:
         # FP8 decode kernel only supports h_q = 64 or 128
@@ -710,19 +710,12 @@ class FlashMLASparseImpl(MLACommonBaseImpl[FlashMLASparseMetadata]):
         indexer: "Indexer | None" = None,
         **mla_args,
     ) -> None:
-        super().__init__(
-            num_heads,
-            head_size,
-            scale,
-            num_kv_heads,
-            alibi_slopes,
-            sliding_window,
-            kv_cache_dtype,
-            logits_soft_cap,
-            attn_type,
-            kv_sharing_target_layer_name,
-            **mla_args,
-        )
+        self.num_heads = num_heads
+        self.head_size = head_size
+        self.scale = float(scale)
+        self.num_kv_heads = num_kv_heads
+        self.kv_cache_dtype = kv_cache_dtype
+        self.kv_lora_rank: int = mla_args["kv_lora_rank"]
         self.softmax_scale = scale
         assert indexer is not None
         self.topk_indices_buffer: torch.Tensor | None = indexer.topk_indices_buffer
@@ -974,78 +967,39 @@ class FlashMLASparseImpl(MLACommonBaseImpl[FlashMLASparseMetadata]):
         output = output[:, : self.num_heads, :]
         return output
 
-    def forward(
+    def forward_mqa(
         self,
+        q: torch.Tensor | tuple[torch.Tensor, torch.Tensor],
+        kv_c_and_k_pe_cache: torch.Tensor,
+        attn_metadata: FlashMLASparseMetadata,
         layer: AttentionLayer,
-        q: torch.Tensor,
-        k_c_normed: torch.Tensor,  # key in unified attn
-        k_pe: torch.Tensor,  # value in unified attn
-        kv_cache: torch.Tensor,
-        attn_metadata: FlashMLASparseMetadata | None,
-        output: torch.Tensor | None = None,
-        output_scale: torch.Tensor | None = None,
-        output_block_scale: torch.Tensor | None = None,
-    ) -> torch.Tensor:
+    ) -> tuple[torch.Tensor, torch.Tensor | None]:
         # NOTE(lucas): for the sparse FlashMLA kernels the kernels want to use
         # MQA 576/512 approach for both prefill and decode
 
-        assert output is not None, "Output tensor must be provided."
-
-        if output_scale is not None or output_block_scale is not None:
-            raise NotImplementedError(
-                "fused output quantization is not yet supported for MLACommonImpl"
-            )
-
-        if attn_metadata is None:
-            # Dummy run - no need to allocate buffers
-            # The zero fill is required when used with DP + EP
-            # to ensure all ranks within a DP group compute the
-            # same expert outputs.
-            return output.fill_(0)
-
-        num_actual_toks = attn_metadata.num_actual_tokens
+        # Concatenate q if it's a tuple (ql_nope, q_pe)
+        if isinstance(q, tuple):
+            q = torch.cat(q, dim=-1)
 
-        # Inputs and outputs may be padded for CUDA graphs
+        num_actual_toks = q.shape[0]
 
-        q = q[:num_actual_toks, ...]
-        k_c_normed = k_c_normed[:num_actual_toks, ...]
-        k_pe = k_pe[:num_actual_toks, ...]
+        # Get topk indices
         assert self.topk_indices_buffer is not None
         topk_indices = self.topk_indices_buffer[:num_actual_toks]
 
-        q_nope, q_pe = q.split([self.qk_nope_head_dim, self.qk_rope_head_dim], dim=-1)
-        # Convert from (B, N, P) to (N, B, P)
-        q_nope = q_nope.transpose(0, 1)
-        # Multiply (N, B, P) x (N, P, L) -> (N, B, L)
-        ql_nope = torch.bmm(q_nope, self.W_UK_T)
-        # Convert from (N, B, L) to (B, N, L)
-        ql_nope = ql_nope.transpose(0, 1)
-
         use_fp8_cache = self.kv_cache_dtype == "fp8_ds_mla"
 
-        q = torch.cat([ql_nope, q_pe], dim=-1)
-
-        # write the latent and rope to kv cache
-        if kv_cache.numel() > 0:
-            ops.concat_and_cache_mla(
-                k_c_normed,
-                k_pe.squeeze(1),
-                kv_cache,
-                attn_metadata.slot_mapping.flatten(),
-                kv_cache_dtype=self.kv_cache_dtype,
-                scale=layer._k_scale,
-            )
-
         if not use_fp8_cache:
-            attn_out = self._forward_bf16_kv(q, kv_cache, topk_indices, attn_metadata)
+            attn_out = self._forward_bf16_kv(
+                q, kv_c_and_k_pe_cache, topk_indices, attn_metadata
+            )
         elif attn_metadata.fp8_use_mixed_batch:
             attn_out = self._forward_fp8_kv_mixed_batch(
-                q, kv_cache, topk_indices, attn_metadata
+                q, kv_c_and_k_pe_cache, topk_indices, attn_metadata
             )
         else:
             attn_out = self._forward_fp8_kv_separate_prefill_decode(
-                q, kv_cache, topk_indices, attn_metadata
+                q, kv_c_and_k_pe_cache, topk_indices, attn_metadata
             )
 
-        self._v_up_proj(attn_out, out=output[:num_actual_toks])
-        return output
+        return attn_out, None

vllm/v1/attention/backends/mla/rocm_aiter_mla.py

@@ -241,7 +241,7 @@ class AiterMLAImpl(MLACommonImpl[AiterMLAMetadata]):
 
         return output
 
-    def _forward_decode(
+    def forward_mqa(
         self,
         q: torch.Tensor | tuple[torch.Tensor, torch.Tensor],
         kv_c_and_k_pe_cache: torch.Tensor,

vllm/v1/attention/backends/mla/rocm_aiter_mla_sparse.py

@@ -7,12 +7,10 @@ from typing import TYPE_CHECKING, ClassVar
 import numpy as np
 import torch
 
-from vllm import _custom_ops as ops
 from vllm._aiter_ops import rocm_aiter_ops
 from vllm.config import VllmConfig
 from vllm.logger import init_logger
 from vllm.model_executor.layers.attention.mla_attention import (
-    MLACommonBaseImpl,
     get_mla_dims,
 )
 from vllm.triton_utils import tl, triton
@@ -23,6 +21,7 @@ from vllm.v1.attention.backend import (
     AttentionMetadata,
     AttentionMetadataBuilder,
     CommonAttentionMetadata,
+    SparseMLAAttentionImpl,
 )
 from vllm.v1.attention.backends.mla.flashmla_sparse import (
     triton_convert_req_index_to_global_index,
@@ -269,7 +268,7 @@ def reference_mla_sparse_prefill(
     return (result, lse)
 
 
-class ROCMAiterMLASparseImpl(MLACommonBaseImpl[ROCMAiterMLASparseMetadata]):
+class ROCMAiterMLASparseImpl(SparseMLAAttentionImpl[ROCMAiterMLASparseMetadata]):
     def __init__(
         self,
         num_heads: int,
@@ -287,23 +286,15 @@ class ROCMAiterMLASparseImpl(MLACommonBaseImpl[ROCMAiterMLASparseMetadata]):
         indexer: "Indexer | None" = None,
         **mla_args,
     ) -> None:
-        super().__init__(
-            num_heads,
-            head_size,
-            scale,
-            num_kv_heads,
-            alibi_slopes,
-            sliding_window,
-            kv_cache_dtype,
-            logits_soft_cap,
-            attn_type,
-            kv_sharing_target_layer_name,
-            **mla_args,
-        )
+        self.num_heads = num_heads
+        self.head_size = head_size
+        self.scale = float(scale)
+        self.num_kv_heads = num_kv_heads
+        self.kv_cache_dtype = kv_cache_dtype
+        self.kv_lora_rank: int = mla_args["kv_lora_rank"]
         self.softmax_scale = scale
         assert indexer is not None
         self.topk_indices_buffer: torch.Tensor | None = indexer.topk_indices_buffer
-        self.is_fp8bmm_enabled = rocm_aiter_ops.is_fp8bmm_enabled()
 
     def _forward_bf16_kv(
         self,
@@ -342,56 +333,23 @@ class ROCMAiterMLASparseImpl(MLACommonBaseImpl[ROCMAiterMLASparseMetadata]):
 
         return output[:, : self.num_heads, :]
 
-    def forward(
+    def forward_mqa(
         self,
-        layer: AttentionLayer,
-        q: torch.Tensor,
-        k_c_normed: torch.Tensor,  # key in unified attn
-        k_pe: torch.Tensor,  # value in unified attn
-        kv_cache: torch.Tensor,
+        q: torch.Tensor | tuple[torch.Tensor, torch.Tensor],
+        kv_c_and_k_pe_cache: torch.Tensor,
         attn_metadata: ROCMAiterMLASparseMetadata,
-        output: torch.Tensor | None = None,
-        output_scale: torch.Tensor | None = None,
-        output_block_scale: torch.Tensor | None = None,
-    ) -> torch.Tensor:
+        layer: AttentionLayer,
+    ) -> tuple[torch.Tensor, torch.Tensor | None]:
         # NOTE(lucas): for the sparse FlashMLA kernels the kernels want to use
         # MQA 576/512 approach for both prefill and decode
 
-        assert output is not None, "Output tensor must be provided."
-
-        if output_scale is not None or output_block_scale is not None:
-            raise NotImplementedError(
-                "fused output quantization is not yet supported for ROCMAiterMLASparse"
-            )
-
-        if attn_metadata is None:
-            # The zero fill is required when used with DP + EP
-            # to ensure all ranks within a DP group compute the
-            # same expert outputs.
-            return output.fill_(0)
-
-        num_actual_toks = attn_metadata.num_actual_tokens
-
-        # Inputs and outputs may be padded for CUDA graphs
-
-        q = q[:num_actual_toks, ...]
-        k_c_normed = k_c_normed[:num_actual_toks, ...]
-        k_pe = k_pe[:num_actual_toks, ...]
-
-        q_nope, q_pe = q.split([self.qk_nope_head_dim, self.qk_rope_head_dim], dim=-1)
-        # Convert from (B, N, P) to (N, B, P)
-        q_nope = q_nope.transpose(0, 1)
-        if self.is_fp8bmm_enabled:
-            # Multiply+Transpose (N, B, P)x(N, P, L)->(N, B, L)->(B, N, L)
-            ql_nope = rocm_aiter_ops.triton_fp8_bmm(
-                q_nope, self.W_K, self.W_K_scale, group_size=128, transpose_bm=True
-            )
-        else:
-            # Multiply (N, B, P) x (N, P, L) -> (N, B, L)
-            ql_nope = torch.bmm(q_nope, self.W_UK_T)
-            # Convert from (N, B, L) to (B, N, L)
-            ql_nope = ql_nope.transpose(0, 1)
+        # Concatenate q if it's a tuple (ql_nope, q_pe)
+        if isinstance(q, tuple):
+            q = torch.cat(q, dim=-1)
 
+        num_actual_toks = q.shape[0]
+
+        # Get topk indices
         assert self.topk_indices_buffer is not None
         topk_indices = self.topk_indices_buffer[:num_actual_toks]
 
@@ -403,22 +361,8 @@ class ROCMAiterMLASparseImpl(MLACommonBaseImpl[ROCMAiterMLASparseMetadata]):
             NUM_TOPK_TOKENS=attn_metadata.topk_tokens,
         )
 
-        q = torch.cat([ql_nope, q_pe], dim=-1)
-
-        # write the latent and rope to kv cache
-        if kv_cache.numel() > 0:
-            ops.concat_and_cache_mla(
-                k_c_normed,
-                k_pe.squeeze(1),
-                kv_cache,
-                attn_metadata.slot_mapping.flatten(),
-                kv_cache_dtype=self.kv_cache_dtype,
-                scale=layer._k_scale,
-            )
-
         attn_out = self._forward_bf16_kv(
-            q, kv_cache, topk_indices_global, attn_metadata
+            q, kv_c_and_k_pe_cache, topk_indices_global, attn_metadata
         )
 
-        self._v_up_proj(attn_out, out=output[:num_actual_toks])
-        return output
+        return attn_out, None

vllm/v1/attention/backends/mla/triton_mla.py

@@ -110,7 +110,7 @@ class TritonMLAImpl(MLACommonImpl[MLACommonMetadata]):
             **kwargs,
         )
 
-    def _forward_decode(
+    def forward_mqa(
         self,
         q: torch.Tensor | tuple[torch.Tensor, torch.Tensor],
         kv_c_and_k_pe_cache: torch.Tensor,