[flashinfer] [kernel] support for fp8 kv cache for trtllm prefill attention (#24197)

Signed-off-by: Xiaozhu <mxz297@gmail.com>

vllm/envs.py

@@ -163,6 +163,7 @@ if TYPE_CHECKING:
     VLLM_ALLOW_CHUNKED_LOCAL_ATTN_WITH_HYBRID_KV_CACHE: bool = False
     VLLM_ENABLE_RESPONSES_API_STORE: bool = False
     VLLM_USE_TRTLLM_ATTENTION: Optional[str] = None
+    VLLM_FLASHINFER_DISABLE_Q_QUANTIZATION: bool = False
     VLLM_HAS_FLASHINFER_CUBIN: bool = False
     VLLM_USE_FLASHINFER_MOE_MXFP4_MXFP8: bool = False
     VLLM_USE_FLASHINFER_MOE_MXFP4_BF16: bool = False
@@ -1155,6 +1156,10 @@ environment_variables: dict[str, Callable[[], Any]] = {
     "VLLM_USE_TRTLLM_ATTENTION":
     lambda: os.getenv("VLLM_USE_TRTLLM_ATTENTION", None),
 
+    # If set to 1, when we use fp8 kv, we do not quantize Q to fp8
+    "VLLM_FLASHINFER_DISABLE_Q_QUANTIZATION":
+    lambda: bool(int(os.getenv("VLLM_FLASHINFER_DISABLE_Q_QUANTIZATION", "0"))),
+
     # If set, it means we pre-downloaded cubin files and flashinfer will
     # read the cubin files directly.
     "VLLM_HAS_FLASHINFER_CUBIN":
@@ -1310,6 +1315,7 @@ def compute_hash() -> str:
         "VLLM_USE_FLASHINFER_MOE_MXFP4_BF16",
         "VLLM_USE_CUDNN_PREFILL",
         "VLLM_USE_TRTLLM_ATTENTION",
+        "VLLM_FLASHINFER_DISABLE_Q_QUANTIZATION",
         "VLLM_ROCM_USE_AITER",
         "VLLM_ROCM_USE_AITER_PAGED_ATTN",
         "VLLM_ROCM_USE_AITER_LINEAR",

vllm/utils/flashinfer.py

@@ -200,11 +200,6 @@ def use_trtllm_attention(
         logger.info_once("Using TRTLLM attention (query is quantized).")
         return True
 
-    # TRTLLM prefill attention does not support FP8 kv cache with
-    # non-quantized query
-    if is_prefill and kv_cache_dtype.startswith("fp8"):
-        return False
-
     # If sinks are being used, we must use TRTLLM attention as it's
     # the only backend that supports them
     if has_sinks:
@@ -353,6 +348,12 @@ def flashinfer_scaled_fp8_mm(
     return output
 
 
+@functools.cache
+def flashinfer_disable_q_quantization() -> bool:
+    """Cache result which only depends on the environment"""
+    return envs.VLLM_FLASHINFER_DISABLE_Q_QUANTIZATION
+
+
 __all__ = [
     "has_flashinfer",
     "flashinfer_trtllm_fp8_block_scale_moe",

vllm/v1/attention/backends/flashinfer.py

@@ -25,7 +25,8 @@ from vllm.model_executor.layers.quantization.utils.quant_utils import (
 from vllm.platforms import current_platform
 from vllm.triton_utils import tl, triton
 from vllm.utils import cdiv, is_pin_memory_available
-from vllm.utils.flashinfer import (supports_trtllm_attention,
+from vllm.utils.flashinfer import (flashinfer_disable_q_quantization,
+                                   supports_trtllm_attention,
                                    use_trtllm_attention)
 from vllm.v1.attention.backends.flash_attn import use_cascade_attention
 # yapf conflicts with isort for this block
@@ -48,8 +49,89 @@ FP4_DTYPE = torch.uint8
 logger = init_logger(__name__)
 
 
-class FlashInferBackend(AttentionBackend):
+@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 FlashInferBackend(AttentionBackend):
     accept_output_buffer: bool = True
 
     @classmethod
@@ -122,7 +204,6 @@ class FlashInferBackend(AttentionBackend):
 
 @dataclass
 class FlashInferMetadata:
-
     num_actual_tokens: int  # Number of tokens excluding padding.
 
     # The data type of the query
@@ -175,8 +256,8 @@ class FlashInferMetadataBuilder(AttentionMetadataBuilder[FlashInferMetadata]):
                                      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
-        self.enable_cuda_graph = self.compilation_config.cudagraph_mode.\
-            decode_mode() == CUDAGraphMode.FULL
+        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.
@@ -201,7 +282,8 @@ class FlashInferMetadataBuilder(AttentionMetadataBuilder[FlashInferMetadata]):
             assert self.kv_cache_spec.dtype == self.model_config.dtype
             self.kv_cache_dtype = self.kv_cache_spec.dtype
 
-        if supports_trtllm_attention()[0]:
+        if supports_trtllm_attention()[0] and \
+            not flashinfer_disable_q_quantization():
             self.q_data_type = self.kv_cache_dtype
         else:
             self.q_data_type = self.model_config.dtype
@@ -795,11 +877,29 @@ class FlashInferImpl(AttentionImpl):
                     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=kv_cache_permute,
+                    kv_cache=mock_kv_cache,
                     workspace_buffer=workspace_buffer,
-                    block_tables=block_tables_prefill,
+                    block_tables=mock_block_table,
                     seq_lens=seq_lens_prefill,
                     max_q_len=attn_metadata.max_q_len,
                     max_kv_len=attn_metadata.max_seq_len,