[1/2] Support FA4 for MHA Prefill in sgl-kernel (#10940)

sgl-kernel/python/sgl_kernel/flash_attn.py

@@ -153,7 +153,43 @@ def flash_attn_with_kvcache(
             normalization factor).
     """
     if ver == 4:
-        raise NotImplementedError("haven't implemented flash_attn_with_kvcache for fa4")
+        assert (
+            flash_attn_varlen_func_v4 is not None
+        ), "FA4 is not available, please check your installation."
+        # Using `(-1, -1)` as no sliding window causes correctness issues for FA4.
+        assert (
+            k is None and v is None
+        ), "FA4 does not support updating KV cache in-place."
+        assert (
+            rotary_cos is None
+            and rotary_sin is None
+            and rotary_interleaved is None
+            and rotary_seqlens is None
+        ), "FA4 does not support rotary embedding."
+        assert (
+            cache_batch_idx is None and cache_leftpad is None
+        ), "FA4 does not support non-consecutive batch indices or left padding."
+        assert (
+            q_descale is None and k_descale is None and v_descale is None
+        ), "FA4 does not support descale."
+
+        if window_size == (-1, -1):
+            window_size = (None, None)
+        return flash_attn_varlen_func_v4(
+            q=q,
+            k=k_cache,
+            v=v_cache,
+            cu_seqlens_q=cu_seqlens_q,
+            seqused_k=cache_seqlens,
+            softmax_scale=softmax_scale,
+            causal=causal,
+            window_size=window_size,
+            softcap=softcap,
+            pack_gqa=pack_gqa,
+            return_softmax_lse=return_softmax_lse,
+            learnable_sink=sinks,
+            page_table=page_table,
+        )
 
     assert k_cache.stride(-1) == 1, "k_cache must have contiguous last dimension"
     assert v_cache.stride(-1) == 1, "v_cache must have contiguous last dimension"

sgl-kernel/python/sgl_kernel/version.py

@@ -1 +1 @@
-__version__ = "0.3.12"
+__version__ = "0.3.13"