vllm.v1.attention.backends.flashinfer ¶

class BatchDCPPrefillWrapper:
    def __init__(
        self,
        workspace_buffer: torch.Tensor | None = None,
    ):
        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,
        prefill_start: int,
        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_cpu,
            paged_kv_indptr_cpu,
            paged_kv_indices,
            paged_kv_last_page_len_cpu[prefill_start:],
            num_qo_heads * dcp_world_size,
            num_kv_heads,
            head_dim,
            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 = cp_lse_ag_out_rs(
            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]
    # Note: Not sure for all platforms,
    # but on Blackwell, only support a page size of
    # 16, 32, 64
    supported_kernel_block_sizes: ClassVar[list[int | MultipleOf]] = [16, 32, 64]
    supported_kv_cache_dtypes: ClassVar[list[CacheDType]] = [
        "auto",
        "fp8",
        "fp8_e4m3",
        "fp8_e5m2",
    ]

    @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() -> 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":
            stride_order = (0, 1, 2, 3, 4)
        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., VLLM_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:
        from vllm.platforms import current_platform

        capability = current_platform.get_device_capability()
        if capability is not None and capability.major == 10:
            return "HND"
        return None

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)
        self.bmm1_scale: float | None = None
        self.bmm2_scale: float | None = None
        self.o_sf_scale: float | None = None

    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, kNvfp4Quant)
        )

    def supports_quant_query_input(self) -> bool:
        if flashinfer_disable_q_quantization():
            return False

        return self.support_trtllm_attn

    # 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

        # 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.prefill_use_trtllm and attn_metadata.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

        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],
                attn_metadata.slot_mapping,
                self.kv_cache_dtype,
                layer._k_scale,
                layer._v_scale,
            )

            # 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_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_decodes = attn_metadata.num_decodes
        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)
        # Regular attention (common case).
        # Decodes are at the front and prefills are at the back.
        if num_prefill_tokens > 0:
            prefill_wrapper = attn_metadata.prefill_wrapper
            prefill_query = query[num_decode_tokens:]
            assert prefill_query.shape[0] == num_prefill_tokens
            assert prefill_wrapper is not None

            if not attn_metadata.prefill_use_trtllm:
                if self.dcp_world_size > 1:
                    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:
                # prefill_query may be non-contiguous
                prefill_query = prefill_query.contiguous()
                workspace_buffer = _get_trtllm_gen_workspace_buffer()
                block_tables_prefill = attn_metadata.block_table_tensor[num_decodes:]
                seq_lens_prefill = attn_metadata.seq_lens[num_decodes:]

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

                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.max_q_len_prefill,
                    max_kv_len=attn_metadata.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.qo_indptr_gpu,
                    cum_seq_lens_kv=attn_metadata.paged_kv_indptr_gpu,
                    window_left=self.window_left,
                    sinks=self.sinks,
                    o_sf_scale=self.o_sf_scale,
                    out=out,
                )

        if num_decode_tokens > 0:
            decode_wrapper = attn_metadata.decode_wrapper
            decode_query = query[:num_decode_tokens]
            assert decode_query.shape[0] == num_decode_tokens
            assert decode_wrapper is not None

            if not attn_metadata.decode_use_trtllm:
                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.dcp_world_size > 1:
                    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] = cp_lse_ag_out_rs(
                        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
                decode_query = decode_query.contiguous()
                workspace_buffer = _get_trtllm_gen_workspace_buffer()
                block_tables_decode = attn_metadata.block_table_tensor[
                    :num_decode_tokens
                ]
                seq_lens_decode = attn_metadata.seq_lens[:num_decode_tokens]

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

                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.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