[Auto Sync] Update flashattention_backend.py (20250922) (#10762)

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Gordon Gustafson <ggustafson@together.ai>

python/sglang/srt/layers/attention/flashattention_backend.py

@@ -692,8 +692,13 @@ class FlashAttentionBackend(AttentionBackend):
         k_descale, v_descale = None, None
         # only use kv scaling if: 1) fp8 kv is explicitly enabled, 2) RadixAttention
         # has corresponding quantization method so that layer.k_scale is not None,
-        # 3) layer.head_dim <= 256 since fa3 kernel require fp16 and bf16 data type in this case.
-        if self.kv_cache_dtype_str != "auto" and layer.head_dim <= 256:
+        # 3) layer.head_dim <= 256 since fa3 kernel require fp16 and bf16 data type in this case,
+        # 4) fa_impl_ver != 4 since fa4 does not currently support fp8 queries and keys.
+        if (
+            self.kv_cache_dtype_str != "auto"
+            and layer.head_dim <= 256
+            and self.fa_impl_ver != 4
+        ):
             if layer.k_scale is not None:
                 descale_shape = (forward_batch.batch_size, layer.tp_k_head_num)
                 k_descale = layer.k_scale.expand(descale_shape)