[diffusion] fix: guard sage attention sm90 bindings (#34107)

Co-authored-by: RunFMe <RunFMe@users.noreply.github.com>
Co-authored-by: Mick <mickjagger19@icloud.com>

python/sglang/multimodal_gen/runtime/platforms/cuda.py

@@ -151,7 +151,26 @@ class _SageAttentionBackendResolver(_CudaAttentionBackendResolver):
     def resolve(cls, platform) -> str | AttentionBackendEnum:
         try:
             from sageattention import sageattn  # noqa: F401
+        except ImportError as e:
+            logger.info(e)
+            logger.info(
+                "Sage Attention backend is not installed (To install it, run `pip install git+https://github.com/thu-ml/SageAttention.git@d9704247a5139ab4c03bf7fc6b35cc0e2cbb5ea4 --no-build-isolation`). Falling back to Flash Attention."
+            )
+            return AttentionBackendEnum.FA
 
+        if platform.is_hopper():
+            try:
+                # fixed SM90 bindings retain the fake implementation under its own name
+                from sageattention.sm90_compile import (  # noqa: F401
+                    qk_int8_sv_f8_accum_f32_fuse_v_scale_attn_inst_buf_fake_impl,
+                )
+            except ImportError:
+                logger.warning(
+                    "Installed Sage Attention is missing the SM90 binding fix. Falling back to Flash Attention. Reinstall with `pip install --force-reinstall git+https://github.com/thu-ml/SageAttention.git@d9704247a5139ab4c03bf7fc6b35cc0e2cbb5ea4 --no-build-isolation`."
+                )
+                return AttentionBackendEnum.FA
+
+        try:
             from sglang.multimodal_gen.runtime.layers.attention.backends.sage_attn import (  # noqa: F401
                 SageAttentionBackend,
             )
@@ -160,7 +179,7 @@ class _SageAttentionBackendResolver(_CudaAttentionBackendResolver):
         except ImportError as e:
             logger.info(e)
             logger.info(
-                "Sage Attention backend is not installed (To install it, run `pip install sageattention==2.2.0 --no-build-isolation`). Falling back to Flash Attention."
+                "Sage Attention backend failed to import. Falling back to Flash Attention."
             )
             return AttentionBackendEnum.FA
 

python/sglang/multimodal_gen/test/unit/test_cuda_attention_backend.py

@@ -1,3 +1,5 @@
+import sys
+import types
 import unittest
 from unittest.mock import patch
 
@@ -6,7 +8,10 @@ import torch
 from sglang.multimodal_gen.runtime.layers.attention.selector import (
     _cached_get_attn_backend,
 )
-from sglang.multimodal_gen.runtime.platforms.cuda import CudaPlatformBase
+from sglang.multimodal_gen.runtime.platforms.cuda import (
+    CudaPlatformBase,
+    _SageAttentionBackendResolver,
+)
 from sglang.multimodal_gen.runtime.platforms.interface import AttentionBackendEnum
 
 SDPA_BACKEND_CLS_STR = (
@@ -17,6 +22,7 @@ SDPA_BACKEND_CLS_STR = (
 class FakeCudaPlatform(CudaPlatformBase):
     is_sm120_device = False
     is_blackwell_device = False
+    is_hopper_device = False
     supports_flash_attention = True
 
     @classmethod
@@ -27,6 +33,10 @@ class FakeCudaPlatform(CudaPlatformBase):
     def is_blackwell(cls):
         return cls.is_blackwell_device
 
+    @classmethod
+    def is_hopper(cls):
+        return cls.is_hopper_device
+
     @classmethod
     def has_device_capability(
         cls,
@@ -40,6 +50,7 @@ class TestCudaAttentionBackendSelection(unittest.TestCase):
     def setUp(self):
         FakeCudaPlatform.is_sm120_device = False
         FakeCudaPlatform.is_blackwell_device = False
+        FakeCudaPlatform.is_hopper_device = False
         FakeCudaPlatform.supports_flash_attention = True
         _cached_get_attn_backend.cache_clear()
 
@@ -112,6 +123,25 @@ class TestCudaAttentionBackendSelection(unittest.TestCase):
         with self.assertRaisesRegex(ValueError, "Invalid attention backend"):
             self.resolve(AttentionBackendEnum.AITER_SAGE)
 
+    def test_hopper_sage_attention_without_sm90_fix_falls_back(self):
+        FakeCudaPlatform.is_hopper_device = True
+        sageattention = types.ModuleType("sageattention")
+        sageattention.__path__ = []
+        sageattention.sageattn = object()
+        sm90_compile = types.ModuleType("sageattention.sm90_compile")
+
+        with patch.dict(
+            sys.modules,
+            {
+                "sageattention": sageattention,
+                "sageattention.sm90_compile": sm90_compile,
+            },
+        ):
+            self.assertEqual(
+                _SageAttentionBackendResolver.resolve(FakeCudaPlatform),
+                AttentionBackendEnum.FA,
+            )
+
     def test_explicit_backend_rejected_by_a_model_fails_closed(self):
         with self.assertRaisesRegex(
             ValueError, "not supported by this attention layer"