[Bugfix][Attention] Resolve kernel block-size constraints per KV cache spec

[xutingl]

Purpose

FlashAttentionBackend.get_supported_kernel_block_sizes() decides its FA4 hd256 page constraint ([128] on SM100/SM110) and, since #53175, its SM90 FP8-KV hd512 constraint ([64]) from the KV cache spec it is given, else from the model-wide ModelConfig. Two of the places that choose block sizes still query it without a spec, so layers whose head size differs from the model-wide one, in practice a spec-decode drafter (DFlash, EAGLE-3, DSpark, MTP) with smaller heads than its target, are held to the target's constraint:

1. the default block size (Platform.update_block_size_for_backend, which since [Bugfix] Pick a KV block size supported by every attention backend #49845 consults every backend found in the loaded layers, including the drafter's);
2. each KV cache group's kernel block size (prepare_kernel_block_sizes / select_common_block_size).

Example: Qwen3.8-27B (GDN hybrid, head size 256, full attention on FlashInfer) + z-lab/Qwen3.8-27B-DFlash2 (head size 128, FlashAttention) on GB200. The Mamba alignment gives an 832-token block, which the drafter's inherited 128-token page constraint does not divide.

• Before [Bugfix] Pick a KV block size supported by every attention backend #49845, KV-cache init failed: ValueError: No common block size for 832. ([Bugfix][Attention] Resolve kernel block sizes per attention group #58425).
• On main today, the default block size is 128 and the Mamba alignment then picks 896 instead of 832: Mamba page padding 9.5% instead of 1.7% and a 4.5% smaller KV cache.

#58207 notes the same failure for Qwen3.6-35B-A3B + DFlash on B300.

Fix

Pass each layer's KV cache spec to the backend at both sites, as _largest_kernel_block_within already does for sliding-window layers since #53175.

• AttentionBackend.supports_block_size(block_size, kv_cache_spec=None) forwards the spec to get_supported_kernel_block_sizes; the overrides only gain the parameter.
• Platform._find_non_ssm_backend_specs collects distinct (backend, spec) pairs from the layers, and _preferred_block_size_for_backends resolves each backend's sizes for the spec it serves. A lone backend keeps its get_preferred_block_size preference; _find_non_ssm_backends and the alignment phases are unchanged.
• select_common_block_size takes optional per-backend specs; prepare_kernel_block_sizes passes each attention group's. Other callers are unchanged.

FlashAttention itself needs no change, and only layers whose spec carries a different constraint than the model-wide one are affected.

Not covered: backend selection under a user --block-size (validate_configuration) receives primitives, not a spec, and still checks the model-wide constraint. In the example above, --block-size 64 therefore still precludes FlashAttention for the drafter, which falls back to an eager FlashInfer draft.

Supersedes #58425 and replaces #58491.

Test Plan

pytest tests/v1/worker/test_gpu_model_runner.py -k "select_common_block_size or preferred_block_size"
pytest tests/kernels/attention/test_attention_selector.py -k fa4_hd256
pytest tests/v1/worker/test_attn_utils.py

E2E on one GB200 (this change applied on main at a1b6763, the last commit before the FlashInfer 0.7.0 bump, which the test environment does not have): Qwen3.8-27B (bf16) + z-lab/Qwen3.8-27B-DFlash2 (num_speculative_tokens=7), default block size; single-request decode benchmark with the acceptance length pinned to 3.

Test Result

Unit tests pass (new: test_preferred_block_size_resolves_constraints_per_spec, test_select_common_block_size_resolves_sizes_per_spec, test_fa4_hd256_block_size_resolves_from_spec).

	main	this PR
block size	896 (Mamba page padding 9.54%)	832 (Mamba page padding 1.71%)
GPU KV cache	578,094 tokens	605,393 tokens
decode TPOT	4.53 ms	4.51 ms

---

Code written with AI assistance (Claude); reviewed, tested and validated end to end by the author.

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @xutingl.

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @xutingl.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork