Allow to use flex_attention instead of FSDPA in HPUAttentionImpl (#876)

Co-authored-by: Michał Kuligowski <mkuligowski@habana.ai>

Author: m-a-nowak

README_GAUDI.md

@@ -373,6 +373,7 @@ Additionally, there are HPU PyTorch Bridge environment variables impacting vLLM
 - `PT_HPU_LAZY_MODE`: if `0`, PyTorch Eager backend for Gaudi will be used, if `1` PyTorch Lazy backend for Gaudi will be used. `1` is the default.
 - `PT_HPU_ENABLE_LAZY_COLLECTIVES` must be set to `true` for tensor parallel inference with HPU Graphs.
 - `PT_HPUGRAPH_DISABLE_TENSOR_CACHE` must be set to `false` for llava model.
+- `VLLM_PROMPT_USE_FLEX_ATTENTION` is enabled only for llama model, and allows to use torch.nn.attention.flex_attention instead of FusedSDPA. Note, this requires `VLLM_PROMPT_USE_FUSEDSDPA=0`
 
 # Quantization, FP8 Inference and Model Calibration Process
 

requirements-hpu.txt

@@ -8,4 +8,4 @@ pandas
 tabulate
 setuptools>=61
 setuptools-scm>=8
-vllm-hpu-extension @ git+https://github.com/HabanaAI/vllm-hpu-extension.git@3fd0250
+vllm-hpu-extension @ git+https://github.com/HabanaAI/vllm-hpu-extension.git@ecb60e4

vllm/attention/backends/hpu_attn.py

@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
 ###############################################################################
-# Copyright (C) 2024 Habana Labs, Ltd. an Intel Company
+# Copyright (C) 2024-2025 Habana Labs, Ltd. an Intel Company
 ###############################################################################
 
 from dataclasses import dataclass
@@ -159,6 +159,8 @@ def __init__(
                 logger().warning("Could not import HPU FusedSDPA kernel. "
                                  "vLLM will use native implementation.")
 
+        self.prefill_use_flex_attention = "flex_attention" in enabled_flags()
+
         suppored_head_sizes = HPUPagedAttention.get_supported_head_sizes()
         if head_size not in suppored_head_sizes:
             raise ValueError(
@@ -237,7 +239,8 @@ def forward(
                         self.head_size)
 
             if attn_metadata is None or attn_metadata.block_list is None:
-                if not self.prefill_use_fusedsdpa:
+                if (not self.prefill_use_fusedsdpa
+                        and not self.prefill_use_flex_attention):
                     # TODO: move this outside of model
                     assert attn_metadata.attn_bias is not None, \
                             'attn_bias must be set before calling model.forward'
@@ -252,20 +255,29 @@ def forward(
                 else:
                     attn_bias = attn_metadata.attn_bias
 
-                out = ops.prompt_attention(
-                    query.view(query_shape),
-                    key.view(kv_shape),
-                    value.view(kv_shape),
-                    attn_bias=attn_bias,
-                    p=0.0,
-                    scale=self.scale,
-                    matmul_qk_op=self.matmul_qk,
-                    softmax_op=self.softmax,
-                    matmul_av_op=self.matmul_av,
-                    valid_seq_lengths=attn_metadata.seq_lens_tensor,
-                    fsdpa_op=self.fused_scaled_dot_product_attention
-                    if self.prefill_use_fusedsdpa else None,
-                )
+                if not self.prefill_use_flex_attention:
+                    out = ops.prompt_attention(
+                        query.view(query_shape),
+                        key.view(kv_shape),
+                        value.view(kv_shape),
+                        attn_bias=attn_bias,
+                        p=0.0,
+                        scale=self.scale,
+                        matmul_qk_op=self.matmul_qk,
+                        softmax_op=self.softmax,
+                        matmul_av_op=self.matmul_av,
+                        valid_seq_lengths=attn_metadata.seq_lens_tensor,
+                        fsdpa_op=self.fused_scaled_dot_product_attention
+                        if self.prefill_use_fusedsdpa else None,
+                    )
+                else:
+                    out = ops.flex_attention(
+                        query.view(query_shape),
+                        key.view(kv_shape),
+                        value.view(kv_shape),
+                        scale=self.scale,
+                    )
+
             else:
                 # TODO: enable FusedSDPA
                 out = HPUPagedAttention.forward_prefix(