[Perf] Use native DSA indexer decode path for next_n > 2 on SM100 (#45322)

Signed-off-by: zixi-qi <zixi@inferact.ai>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: Yongye Zhu <zyy1102000@gmail.com>

vllm/v1/attention/backends/mla/indexer.py

@@ -231,8 +231,6 @@ def get_max_prefill_buffer_size(vllm_config: VllmConfig):
 
 class DeepseekV32IndexerMetadataBuilder(AttentionMetadataBuilder):
     reorder_batch_threshold: int = 1
-    natively_supported_next_n_fp4: list[int] = [1, 2]
-    # TODO (matt): integrate kernel with next_n = 4 support
 
     @classmethod
     def get_cudagraph_support(
@@ -267,15 +265,21 @@ class DeepseekV32IndexerMetadataBuilder(AttentionMetadataBuilder):
 
         next_n = self.num_speculative_tokens + 1
         self.reorder_batch_threshold += self.num_speculative_tokens
-        # NOTE(zyongye) fp4 indexer cache only natively supports next_n in
-        # natively_supported_next_n_fp4; for other next_n values we fall back
-        # to the flattening path. Outside the SM100 datacenter family the FP8
-        # paged MQA logits kernel has the same [1, 2] constraint (deepgemm
-        # smxx_fp8_fp4_paged_mqa_logits.hpp:233), so flatten there too.
-        self.use_flattening = (
-            self.use_fp4_indexer_cache
-            or not current_platform.is_device_capability_family(100)
-        ) and next_n not in self.natively_supported_next_n_fp4
+        # NOTE: SM100 datacenter GPUs support any next_n natively via the
+        # multi-atom paged MQA logits kernels (FP8 and FP4 indexer
+        # caches). Outside the SM100 family the FP8
+        # paged MQA logits kernel only supports next_n in (1, 2)
+        # (deepgemm smxx_fp8_fp4_paged_mqa_logits.hpp:233), so flatten there.
+        self.use_flattening = not current_platform.is_device_capability_family(
+            100
+        ) and next_n not in (1, 2)
+        logger.info_once(
+            "DSA indexer decode path: use_flattening=%s "
+            "(next_n=%d, use_fp4_indexer_cache=%s)",
+            self.use_flattening,
+            next_n,
+            self.use_fp4_indexer_cache,
+        )
 
         sm_count = num_compute_units(self.device.index)
         self.num_sms = sm_count