vllm.v1.attention.backends.flashinfer ¶

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)

    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_tuple: tuple[torch.Tensor, 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_tuple,
            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

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

    wrapper: BatchDecodeWithPagedKVCacheWrapper

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

    wrapper: BatchPrefillWithPagedKVCacheWrapper | BatchDCPPrefillWrapper

class FlashInferBackend(AttentionBackend):
    supported_dtypes: ClassVar[list[torch.dtype]] = [torch.float16, torch.bfloat16]
    supported_kv_cache_dtypes: ClassVar[list[CacheDType]] = [
        "auto",
        "float16",
        "bfloat16",
        "fp8",
        "fp8_e4m3",
        "fp8_e5m2",
        "nvfp4",
    ]

    @staticmethod
    def get_supported_kernel_block_sizes() -> list[int | MultipleOf]:
        # Page sizes >= 128 only run on the trtllm-gen dynamic kernel (GQA/MQA
        # on Blackwell); advertise them only when usable so selection never
        # picks a large kernel block we cannot serve.
        use_large_pages = False
        vllm_config = get_current_vllm_config_or_none()
        if vllm_config is not None and vllm_config.model_config is not None:
            pc = vllm_config.parallel_config
            mc = vllm_config.model_config
            num_qo_heads = mc.get_num_attention_heads(pc)
            num_kv_heads = mc.get_num_kv_heads(pc)
            use_large_pages = (
                num_kv_heads > 0
                and num_qo_heads // num_kv_heads > 1
                and current_platform.is_device_capability_family(100)
                and can_use_trtllm_attention(num_qo_heads, num_kv_heads)
            )
        if not use_large_pages:
            return [16, 32, 64]
        return [16, 32, 64, 128, 256, 512, 1024]

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

    @classmethod
    def supports_non_causal(cls) -> bool:
        return True

    @classmethod
    def supports_sliding_window(cls) -> bool:
        return True

    @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, ...]:
        if cache_dtype_str == "nvfp4":
            full_dim = nvfp4_kv_cache_full_dim(head_size)
            return (num_blocks, 2 * num_kv_heads, block_size, full_dim)
        # Pack K and V in the content dim (B, H, N, 2*hs).
        return (num_blocks, num_kv_heads, block_size, 2 * 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` (logical (B, H, N, 2*hs)) 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, block_size, num_kv_heads, 2*head_size)
            return (1, 0, 3, 2, 4)
        elif cache_layout == "NHD":
            # (num_blocks, block_size, num_kv_heads, 2*head_size)
            stride_order = (0, 2, 1, 3)
        elif cache_layout == "HND" and include_num_layers_dimension:
            # (num_blocks, num_kv_heads, num_layers, block_size, 2*head_size)
            return (1, 2, 0, 3, 4)
        elif cache_layout == "HND":
            # (num_blocks, num_kv_heads, block_size, 2*head_size)
            stride_order = (0, 1, 2, 3)
        else:
            raise ValueError(f"Unknown cache layout format {cache_layout}.")
        return stride_order

    @staticmethod
    def get_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
        elif kv_cache_dtype == "nvfp4":
            return torch.uint8
        else:
            raise ValueError(f"Unrecognized dtype: {kv_cache_dtype}")

    @classmethod
    def supports_kv_cache_dtype(cls, kv_cache_dtype: CacheDType | None) -> bool:
        if kv_cache_dtype == "nvfp4":
            return (
                current_platform.is_device_capability_family(100)
                and supports_trtllm_attention(is_prefill=True)
                and supports_trtllm_attention(is_prefill=False)
            )
        return super().supports_kv_cache_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, 512]

    @classmethod
    def supports_compute_capability(cls, capability: DeviceCapability) -> bool:
        # FlashInfer supports SM75+, but is currently broken on SM75 (Turing):
        # https://github.com/flashinfer-ai/flashinfer/issues/3620 (fix:
        # https://github.com/flashinfer-ai/flashinfer/pull/3621). Temporarily
        # raise the floor to SM80 so it is not auto-selected on SM75 until
        # that fix lands; revert to DeviceCapability(7, 5) once it does.
        return capability >= DeviceCapability(8, 0) and capability <= DeviceCapability(
            12, 1
        )

    @classmethod
    def supports_sink(cls) -> bool:
        """FlashInfer supports sinks only on the SM100 trtllm-gen path."""
        from vllm.utils.flashinfer import (
            force_use_trtllm_attention,
        )

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

        if not current_platform.is_device_capability_family(100):
            return False

        # Check if TRTLLM is supported on this platform
        return supports_trtllm_attention(
            is_prefill=False
        ) and supports_trtllm_attention(is_prefill=True)

    @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

class FlashInferDecodeKernel(Enum):
    """Decode kernels selected inside the FlashInfer backend."""

    XQA = "xqa"
    TRTLLM_GEN = "trtllm-gen"

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.is_kvcache_nvfp4 = kv_cache_dtype == "nvfp4"
        self.fp4_data_dim = head_size // 2 if self.is_kvcache_nvfp4 else 0
        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.supports_xqa_or_trtllm_gen_decode = can_use_trtllm_attention(
            num_heads, num_kv_heads, is_prefill=False
        )
        vllm_config = get_current_vllm_config_or_none()
        # Query pre-quantization needs a single dtype for the whole query tensor.
        # SM90 XQA needs BF16/FP16-Q for decode and FP8 for prefill,
        # so only enable this for SM100 trtllm-gen where both use FP8-Q.
        self.supports_quant_query_input = (
            self.supports_xqa_or_trtllm_gen_decode
            and is_quantized_kv_cache(self.kv_cache_dtype)
            and current_platform.is_device_capability_family(100)
            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

        # Pre-allocated FP8 output buffer for NVFP4 without fused output quant.
        if self.is_kvcache_nvfp4 and vllm_config is not None:
            max_num_tokens = vllm_config.scheduler_config.max_num_batched_tokens
            self._nvfp4_fp8_out = torch.empty(
                (max_num_tokens, num_heads, head_size),
                dtype=FP8_DTYPE,
                device="cuda",
            )
        else:
            self._nvfp4_fp8_out = 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):
        # XQA does not support FP8/NVFP4 output, so require trtllm-gen
        # (SM100+) here.  Without that we cannot fuse the output quant.
        return (
            self.supports_xqa_or_trtllm_gen_decode
            and is_quantized_kv_cache(self.kv_cache_dtype)
            and current_platform.is_device_capability_family(100)
            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 get_xqa_bmm1_scale(self, layer: torch.nn.Module, q_data_type: torch.dtype):
        bmm1_scale = self.scale
        if is_quantized_kv_cache(self.kv_cache_dtype):
            if q_data_type in (torch.float8_e4m3fn, torch.float8_e5m2):
                bmm1_scale *= layer._q_scale_float
            bmm1_scale *= layer._k_scale_float
        return bmm1_scale

    # SM90 may need FP8-Q for native prefill and BF16/FP16-Q for XQA decode,
    # so quantize only the slice whose target dtype differs.
    def maybe_quant_query(
        self,
        query: torch.Tensor,
        q_data_type: torch.dtype,
        scale: torch.Tensor,
    ) -> torch.Tensor:
        if query.dtype != q_data_type:
            assert query.dtype in [torch.float16, torch.bfloat16]
            assert q_data_type in [torch.float8_e4m3fn, torch.float8_e5m2]
            assert query.dim() == 3
            num_tokens = query.shape[0]
            num_heads = query.shape[1]
            head_size = query.shape[2]
            assert query.stride(2) == 1 and query.stride(1) == head_size
            query_quantized, _ = custom_ops.scaled_fp8_quant(
                query.view(num_tokens, num_heads * head_size), scale=scale
            )
            return query_quantized.view(num_tokens, num_heads, head_size)

        return query

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

        if self.bmm1_scale is None:
            self.bmm1_scale = self.scale
            if is_quantized_kv_cache(self.kv_cache_dtype):
                self.bmm1_scale *= layer._q_scale_float * layer._k_scale_float

        if self.bmm2_scale is None:
            self.bmm2_scale = 1.0
            if is_quantized_kv_cache(self.kv_cache_dtype):
                self.bmm2_scale *= layer._v_scale_float

        prefill_use_trtllm = isinstance(attn_metadata.prefill, TRTLLMPrefill)
        decode_kernel = (
            attn_metadata.decode.kernel
            if isinstance(attn_metadata.decode, FlashInferTrtllmAPIDecode)
            else None
        )
        decode_with_xqa = decode_kernel == FlashInferDecodeKernel.XQA
        decode_with_trtllm_gen = decode_kernel == FlashInferDecodeKernel.TRTLLM_GEN
        decode_with_flashinfer_trtllm_api = decode_with_xqa or decode_with_trtllm_gen

        # 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_prefill == FP8_DTYPE, (
                "Query must be FP8 when attn+quant fusion happened for prefill."
            )
            assert attn_metadata.q_data_type_decode == FP8_DTYPE, (
                "Query must be FP8 when attn+quant fusion happened for decode."
            )
            assert (attn_metadata.num_prefills == 0 or prefill_use_trtllm) and (
                attn_metadata.num_decodes == 0 or decode_with_trtllm_gen
            ), "Output quant fusion requires TRTLLM prefill/trtllm-gen decode"

            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

        # FlashInfer treats uint8 KV cache as NVFP4. vLLM stores FP8 KV cache
        # as uint8 bytes, so pass FP8 caches with their logical dtype.
        if not self.is_kvcache_nvfp4 and kv_cache.dtype == torch.uint8:
            fp8_view_dtype = None
            if self.kv_cache_dtype in ("fp8", "fp8_e4m3", torch.float8_e4m3fn):
                fp8_view_dtype = torch.float8_e4m3fn
            elif self.kv_cache_dtype in ("fp8_e5m2", torch.float8_e5m2):
                fp8_view_dtype = torch.float8_e5m2
            if fp8_view_dtype is not None:
                kv_cache = kv_cache.view(fp8_view_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
            stride_order = FlashInferBackend.get_kv_cache_stride_order()
            if self.is_kvcache_nvfp4:
                kv_cache_views = tuple(
                    cache.permute(*stride_order)
                    for cache in kv_cache.split(self.num_kv_heads, dim=1)
                )
            else:
                kv_perm = kv_cache.permute(*stride_order)
                kv_cache_views = kv_perm.split(self.head_size, dim=-1)
            kv_tuple = tuple(
                canonicalize_singleton_dim_strides(cache) for cache in kv_cache_views
            )
            output.copy_(attn_metadata.cascade_wrapper.run(query, kv_tuple))
            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)  # HND and contiguous
        # Fix degenerate strides on any size-1 dimension (e.g. num_kv_heads=1
        # with TP=8).  PyTorch permits non-canonical strides on size-1 dims;
        # CUDA TMA requires ≥16-byte alignment on all non-outermost strides.
        # canonicalize_singleton_dim_strides patches metadata via as_strided —
        # zero-copy.  See vllm.utils.torch_utils.
        fixed = canonicalize_singleton_dim_strides(kv_cache_permute)
        if fixed is not kv_cache_permute:
            logger.debug(
                "Canonicalized degenerate KV cache strides (FlashInfer): "
                "shape=%s, strides before=%s, strides after=%s",
                kv_cache_permute.shape,
                kv_cache_permute.stride(),
                fixed.stride(),
            )
        kv_cache_permute = fixed

        # Split K/V — zero-copy views. NVFP4 stores K/V as separate head
        # groups; other dtypes pack K/V in the content dim.
        hs = self.head_size
        nvfp4_kv_data = None
        nvfp4_kv_block_scales = None
        if self.is_kvcache_nvfp4:
            k_cache, v_cache = kv_cache.split(self.num_kv_heads, dim=1)
            kv_cache_tuple = (
                canonicalize_singleton_dim_strides(k_cache.permute(*stride_order)),
                canonicalize_singleton_dim_strides(v_cache.permute(*stride_order)),
            )
            k_data, k_sf = nvfp4_split_data_scale(kv_cache_tuple[0])
            v_data, v_sf = nvfp4_split_data_scale(kv_cache_tuple[1])
            nvfp4_kv_data = (k_data, v_data)
            nvfp4_kv_block_scales = (k_sf, v_sf)
        else:
            kv_cache_tuple = kv_cache_permute.split(hs, dim=-1)

        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

            # Convert query to the expected dtype for prefill if needed.
            prefill_query = self.maybe_quant_query(
                prefill_query,
                attn_metadata.q_data_type_prefill,
                layer._q_scale,
            )

            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_tuple,
                        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 == attn_metadata.causal

                    if self.is_kvcache_nvfp4:
                        kv_cache_for_fi = nvfp4_kv_data
                    else:
                        kv_cache_for_fi = kv_cache_tuple
                    kv_cache_sf = (
                        nvfp4_kv_block_scales if self.is_kvcache_nvfp4 else None
                    )

                    # NVFP4 trtllm kernel only supports FP8 output.
                    # Use a pre-allocated FP8 buffer and dequantize
                    # afterwards.
                    needs_fp8_out_prefill = (
                        self.is_kvcache_nvfp4 and output.dtype != FP8_DTYPE
                    )
                    if needs_fp8_out_prefill:
                        out_prefill = self._nvfp4_fp8_out[:num_prefill_tokens]
                    else:
                        out_prefill = output[num_decode_tokens:]

                    prefill_wrapper.run(
                        prefill_query,
                        kv_cache_for_fi,
                        q_scale=layer._q_scale_float,
                        k_scale=layer._k_scale_float,
                        v_scale=layer._v_scale_float,
                        out=out_prefill,
                        kv_cache_sf=kv_cache_sf,
                    )

                    if needs_fp8_out_prefill:
                        output[
                            num_decode_tokens : num_decode_tokens + num_prefill_tokens
                        ].copy_(out_prefill.to(output.dtype))
            else:
                assert isinstance(attn_metadata.prefill, TRTLLMPrefill)
                # prefill_query may be non-contiguous or have degenerate strides
                # on size=1 dims. contiguous() ensures memory layout; then
                # canonicalize_singleton_dim_strides fixes any remaining
                # degenerate strides on size=1 dims for TMA alignment.
                prefill_query = prefill_query.contiguous()
                prefill_query = canonicalize_singleton_dim_strides(prefill_query)
                workspace_buffer = _get_trtllm_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(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:]

                # NVFP4 trtllm kernel only supports FP8 output.
                # Use a pre-allocated FP8 buffer and dequantize afterwards.
                needs_fp8_out = self.is_kvcache_nvfp4 and output.dtype != FP8_DTYPE
                if needs_fp8_out:
                    out = self._nvfp4_fp8_out[:num_prefill_tokens]

                prefill_kv_block_scales = None
                if self.is_kvcache_nvfp4:
                    # NVFP4 trtllm-gen kernel requires FP8 query.
                    assert attn_metadata.q_data_type_prefill == FP8_DTYPE, (
                        "NVFP4 KV cache requires FP8 quantized queries for "
                        "trtllm-gen prefill. Set "
                        "disable_flashinfer_q_quantization=False."
                    )
                    mock_kv_cache = nvfp4_kv_data
                    mock_block_table = block_tables_prefill
                    prefill_kv_block_scales = nvfp4_kv_block_scales
                elif (
                    attn_metadata.q_data_type_prefill != 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.
                    kv_cache_permute = canonicalize_singleton_dim_strides(
                        kv_cache_permute
                    )
                    kv_strides = kv_cache_permute.stride()
                    assert (
                        kv_strides[-1] == 1
                        and kv_strides[-2] == kv_cache_permute.shape[-1]
                    ), (
                        "KV cache inner dims (block_size, head_size) must be "
                        f"contiguous, got strides {kv_strides}"
                    )
                    # fp8 uses (B, H, N, 2*hs); reshape to (B, 2, H, N, hs)
                    # for the dequant kernel — zero-copy view. The dequant
                    # kernel handles the interleaved K/V block stride.
                    B_kv, H_kv, N_kv = kv_cache_permute.shape[:3]
                    kv_cache_5d = kv_cache_permute.view(B_kv, H_kv, N_kv, 2, hs)
                    kv_cache_5d = kv_cache_5d.permute(0, 3, 1, 2, 4)
                    mock_kv_cache, mock_block_table = trtllm_prefill_attn_kvfp8_dequant(
                        kv_cache_5d,
                        block_tables_prefill,
                        layer._k_scale,
                        layer._v_scale,
                        attn_metadata.q_data_type_prefill,
                    )
                else:
                    mock_kv_cache = kv_cache_tuple
                    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,
                    kv_cache_sf=prefill_kv_block_scales,
                )

                if needs_fp8_out:
                    output[
                        num_decode_tokens : num_decode_tokens + num_prefill_tokens
                    ].copy_(out[:num_prefill_tokens].to(output.dtype))

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

            # Convert query to the expected dtype for decode if needed.
            decode_query = self.maybe_quant_query(
                decode_query,
                attn_metadata.q_data_type_decode,
                layer._q_scale,
            )

            if not decode_with_flashinfer_trtllm_api:
                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 self.is_kvcache_nvfp4:
                    kv_cache_for_fi = nvfp4_kv_data
                else:
                    kv_cache_for_fi = kv_cache_tuple
                kv_cache_sf = nvfp4_kv_block_scales if self.is_kvcache_nvfp4 else None

                # NVFP4 kernel only supports FP8 output.
                # Use a pre-allocated FP8 buffer and dequantize afterwards.
                needs_fp8_out = self.is_kvcache_nvfp4 and output.dtype != FP8_DTYPE
                if needs_fp8_out:
                    out_decode = self._nvfp4_fp8_out[:num_decode_tokens]
                else:
                    out_decode = output[:num_decode_tokens]

                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_for_fi,
                        q_scale=layer._q_scale_float,
                        k_scale=layer._k_scale_float,
                        v_scale=layer._v_scale_float,
                        out=output_tmp,
                        lse=lse,
                        return_lse=True,
                        kv_cache_sf=kv_cache_sf,
                    )
                    output[:num_decode_tokens] = self.dcp_combine(
                        output_tmp,
                        lse,
                        get_dcp_group(),
                    )
                else:
                    decode_wrapper.run(
                        decode_query,
                        kv_cache_for_fi,
                        q_scale=layer._q_scale_float,
                        k_scale=layer._k_scale_float,
                        v_scale=layer._v_scale_float,
                        out=out_decode,
                        kv_cache_sf=kv_cache_sf,
                    )

                if needs_fp8_out:
                    output[:num_decode_tokens].copy_(out_decode.to(output.dtype))
            else:
                assert isinstance(attn_metadata.decode, FlashInferTrtllmAPIDecode)
                # decode_query may be non-contiguous or have degenerate strides
                # on size=1 dims. contiguous() ensures memory layout; then
                # canonicalize_singleton_dim_strides fixes any remaining
                # degenerate strides on size=1 dims for TMA alignment.
                decode_query = decode_query.contiguous()
                decode_query = canonicalize_singleton_dim_strides(decode_query)
                workspace_buffer = _get_trtllm_workspace_buffer()
                block_tables_decode = attn_metadata.decode.block_tables
                seq_lens_decode = attn_metadata.decode.seq_lens

                # trtllm-gen needs HND layout on SM100. XQA is selected
                # separately on SM90 and does not use this SM100 layout gate.
                if decode_with_trtllm_gen:
                    assert get_kv_cache_layout() == "HND"
                else:
                    assert decode_with_xqa
                assert is_strictly_contiguous(decode_query)
                assert is_strictly_contiguous(workspace_buffer)
                assert is_strictly_contiguous(block_tables_decode)
                assert is_strictly_contiguous(seq_lens_decode)
                kv_cache_permute = canonicalize_singleton_dim_strides(kv_cache_permute)
                kv_strides = kv_cache_permute.stride()
                assert (
                    kv_strides[-1] == 1 and kv_strides[-2] == kv_cache_permute.shape[-1]
                ), (
                    "KV cache inner dims (block_size, head_size) must be "
                    f"contiguous, got strides {kv_strides}"
                )

                if use_dcp:
                    assert decode_with_trtllm_gen
                    if output.dtype == FP4_DTYPE:
                        raise NotImplementedError(
                            "DCP decode with FlashInfer trtllm-gen does not support "
                            "FP4 attention output yet."
                        )
                    decode_query = get_dcp_group().all_gather(
                        decode_query.contiguous(), dim=-2
                    )
                    decode_query = canonicalize_singleton_dim_strides(decode_query)

                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]

                # NVFP4 trtllm kernel only supports FP8 output.
                # Use a pre-allocated FP8 buffer and dequantize afterwards.
                needs_fp8_out = self.is_kvcache_nvfp4 and output.dtype != FP8_DTYPE
                if needs_fp8_out:
                    out = self._nvfp4_fp8_out[: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

                if decode_with_xqa and q_len_per_req > 1:
                    raise NotImplementedError(
                        "FlashInfer XQA speculative decode is not wired in vLLM yet."
                    )

                # XQA decode can use model-dtype Q with FP8 KV, so only include
                # q_scale when the decode query is actually FP8.
                bmm1_scale = (
                    self.get_xqa_bmm1_scale(layer, attn_metadata.q_data_type_decode)
                    if decode_with_xqa
                    else self.bmm1_scale
                )

                lse = None
                if use_dcp:
                    out = torch.empty(
                        decode_query.shape,
                        dtype=output.dtype,
                        device=decode_query.device,
                    )
                    lse = torch.empty(
                        (decode_query.size(0), decode_query.size(1)),
                        dtype=torch.float32,
                        device=decode_query.device,
                    )

                trtllm_batch_decode_with_kv_cache(
                    query=decode_query,
                    kv_cache=(
                        nvfp4_kv_data if self.is_kvcache_nvfp4 else kv_cache_tuple
                    ),
                    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=bmm1_scale,
                    bmm2_scale=self.bmm2_scale,
                    window_left=self.window_left,
                    sinks=self.sinks,
                    o_sf_scale=self.o_sf_scale,
                    out=out,
                    kv_layout=get_kv_cache_layout(),
                    backend=attn_metadata.decode.kernel.value,
                    q_len_per_req=q_len_per_req,
                    kv_cache_sf=(
                        nvfp4_kv_block_scales if self.is_kvcache_nvfp4 else None
                    ),
                    lse=lse,
                    return_lse=self.need_to_return_lse_for_decode,
                )

                if use_dcp:
                    assert isinstance(out, torch.Tensor)
                    assert lse is not None
                    output[:num_decode_tokens] = self.dcp_combine(
                        out,
                        lse,
                        get_dcp_group(),
                    )
                elif needs_fp8_out:
                    output[:num_decode_tokens].copy_(out.to(output.dtype))
        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.
            if self.is_kvcache_nvfp4:
                # (B, 2*H, N, full_dim) -> ((B, N, H, full_dim),
                #                            (B, N, H, full_dim));
                # K heads first, then V heads.
                k_cache, v_cache = kv_cache.transpose(1, 2).split(
                    self.num_kv_heads, dim=-2
                )
            else:
                # (B, H, N, 2*hs) -> ((B, N, H, hs), (B, N, H, hs))
                k_cache, v_cache = kv_cache.transpose(1, 2).split(
                    self.head_size, dim=-1
                )
            torch.ops._C_cache_ops.reshape_and_cache_flash(
                key,
                value,
                k_cache,
                v_cache,
                slot_mapping,
                self.kv_cache_dtype,
                layer._k_scale,
                layer._v_scale,
            )