feat: add FA4 SM90 paged KV decode support & update attention docs (#18442)

Co-authored-by: Zeyu Wang <zeyu.wang@yahooinc.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Brayden Zhong <b8zhong@uwaterloo.ca>

docs/advanced_features/attention_backend.md

@@ -49,10 +49,14 @@ Multimodal attention is selected by `--mm-attention-backend`. The "MultiModal" c
 ```
 
 ```{note}
-- FlashAttention 4 is prefill-only for now.
+- FlashAttention 4 supports both prefill and decode on SM90 (Hopper) and SM100 (Blackwell). On SM90, `page_size` must be 128.
 - NSA is specifically designed for [DeepSeek V3.2 DSA](https://lmsys.org/blog/2025-09-29-deepseek-V32/).
 ```
 
+```{warning}
+**FA4 on Hopper (SM90):** FA4 decode speed decreases as sequence length grows due to lack of SplitKV support. At batch=1 compared to FA3 on H100: ~-10% at 2K tokens, ~-18% at 4K, ~-31% at 8K, ~-49% at 16K. Larger batch sizes reduce the gap (e.g., batch=8: ~-2% at 2K, ~-8% at 4K). Blackwell (SM100) is not affected.
+```
+
 ```{note}
 For the KV4 FA4 scenario, FA4 requires using a different --decode-attention-backend to run. Except for trtllm_mha being incompatible with FA4, all other decode backends behave as shown in the table.
 ```
@@ -204,6 +208,13 @@ python3 -m sglang.launch_server \
 
 - FlashAttention 4 (MHA & MLA)
 ```bash
+# FA4 for both prefill and decode on SM90/SM100
+python3 -m sglang.launch_server \
+  --model-path Qwen/Qwen3-30B-A3B-Instruct-2507-FP8 \
+  --attention-backend fa4 \
+  --page-size 128 \
+  --trust-remote-code
+
 python3 -m sglang.launch_server \
   --tp 8 \
   --model deepseek-ai/DeepSeek-R1 \