[BUGFIX][ROCM] ViT FlashAttention on ROCm (no GFX9) and contiguous on qwen3vl...

[BUGFIX][ROCM] ViT FlashAttention on ROCm (no GFX9) and contiguous on qwen3vl ROCm TORCH_SDPA (#27190)
Signed-off-by: JartX <sagformas@epdcenter.es>
Co-authored-by: tjtanaa <tunjian.tan@embeddedllm.com>

vllm/attention/layer.py

@@ -47,6 +47,12 @@ from vllm.v1.kv_cache_interface import (
     SlidingWindowSpec,
 )
 
+if current_platform.is_rocm():
+    from vllm.platforms.rocm import on_gfx9
+else:
+    on_gfx9 = lambda *args, **kwargs: False
+
+
 FP8_DTYPE = current_platform.fp8_dtype()
 logger = init_logger(__name__)
 USE_XFORMERS_OPS = None
@@ -96,18 +102,29 @@ def maybe_get_vit_flash_attn_backend(
     attn_backend: _Backend,
     use_upstream_fa: bool,
     attn_backend_override: _Backend | None = None,
-) -> tuple[_Backend, Callable]:
-    if (
-        attn_backend != _Backend.FLASH_ATTN
-        and attn_backend != _Backend.ROCM_AITER_FA
-        and check_upstream_fa_availability(torch.get_default_dtype())
+) -> tuple[_Backend, Callable | None]:
+    if current_platform.is_rocm():
+        if envs.VLLM_ROCM_USE_AITER and envs.VLLM_ROCM_USE_AITER_MHA and on_gfx9():
+            attn_backend = _Backend.ROCM_AITER_FA
+
+        elif (
+            check_upstream_fa_availability(torch.get_default_dtype())
+            and on_gfx9()
             and attn_backend_override is None
         ):
             attn_backend = _Backend.FLASH_ATTN
             use_upstream_fa = True
+        else:
+            return _Backend.TORCH_SDPA, None
 
-    if current_platform.is_rocm() and attn_backend == _Backend.FLASH_ATTN:
+    elif current_platform.is_cuda():
+        if attn_backend != _Backend.FLASH_ATTN and check_upstream_fa_availability(
+            torch.get_default_dtype()
+        ):
+            attn_backend = _Backend.FLASH_ATTN
             use_upstream_fa = True
+    else:
+        return _Backend.TORCH_SDPA, None
 
     if attn_backend in {_Backend.FLASH_ATTN, _Backend.ROCM_AITER_FA}:
         if attn_backend == _Backend.ROCM_AITER_FA:
@@ -570,6 +587,7 @@ class MultiHeadAttention(nn.Module):
             value = torch.repeat_interleave(value, num_repeat, dim=2)
 
         if self.is_flash_attn_backend:
+            assert self._flash_attn_varlen_func is not None
             cu_seqlens_q = torch.arange(
                 0, (bsz + 1) * q_len, step=q_len, dtype=torch.int32, device=query.device
             )

vllm/model_executor/models/qwen2_5_vl.py

@@ -429,6 +429,12 @@ class Qwen2_5_VisionAttention(nn.Module):
             ).contiguous()
         elif self.attn_backend == _Backend.TORCH_SDPA:
             # Execute attention entry by entry for speed & less VRAM.
+            from vllm.platforms import current_platform
+
+            if current_platform.is_rocm():
+                q = q.contiguous()
+                k = k.contiguous()
+                v = v.contiguous()
             outputs = []
             for i in range(1, len(cu_seqlens)):
                 start_idx = cu_seqlens[i - 1]

vllm/model_executor/models/qwen2_vl.py

@@ -462,6 +462,12 @@ class Qwen2VisionAttention(nn.Module):
             ).contiguous()
         elif self.attn_backend == _Backend.TORCH_SDPA:
             # Execute attention entry by entry for speed & less VRAM.
+            from vllm.platforms import current_platform
+
+            if current_platform.is_rocm():
+                q = q.contiguous()
+                k = k.contiguous()
+                v = v.contiguous()
             outputs = []
             for i in range(1, len(cu_seqlens)):
                 start_idx = cu_seqlens[i - 1]

vllm/platforms/rocm.py

@@ -205,12 +205,16 @@ class RocmPlatform(Platform):
 
     @classmethod
     def get_vit_attn_backend(cls, head_size: int, dtype: torch.dtype) -> "_Backend":
+        from importlib.util import find_spec
+
         from vllm.attention.backends.registry import _Backend
 
         if envs.VLLM_ROCM_USE_AITER and envs.VLLM_ROCM_USE_AITER_MHA and on_gfx9():
             return _Backend.ROCM_AITER_FA
-        if on_gfx9():
+
+        if on_gfx9() and find_spec("flash_attn") is not None:
             return _Backend.FLASH_ATTN
+
         return _Backend.TORCH_SDPA
 
     @classmethod