[Bugfix][SM120][MLA] Support NoPE sparse MLA (GLM-5.3-Flash) on the FlashInfer SM120 backend

[lucifer1004]

Purpose

Make the FLASHINFER_MLA_SPARSE_SM120 backend usable for GLM-5.3-Flash (glm5_next, #53906), whose sparse MLA layers are NoPE (qk_rope_head_dim=0) with a kpool-widened top-k buffer. Two independent breakages are fixed:

1. concat_and_cache fp8_ds_mla rejects NoPE rows. The packed fp8_ds_mla row layout is unchanged for NoPE models (512B latent + 16B tile scales + 128B reserved RoPE/padding = 656B), but the host check required pe_dim == 64 and the kernel's RoPE warp assumed 64 tail elements. Relax the check to pe_dim ∈ {64, 0} and have the RoPE warp zero the reserved bytes when pe_dim == 0, so consumers of the DS-shaped row (e.g. cp_gather_and_upconvert_fp8_kv_cache on the chunked-context path) read exact zeros rather than stale memory — a zero rope lane is bit-exact NoPE for q_pe · k_pe. The FlashInfer GLM53_NOPE kernels never read the tail either way. A sentinel-filled regression test covers the NoPE row layout.

2. Sparse page-table sized by topk_tokens instead of the actual buffer width. GLM-5.3-Flash widens the top-k buffer past index_topk with a kpool tail (2048 → 2176, rounded to a multiple of 128), so topk_indices_physical.shape[1] != attn_metadata.topk_tokens and FlashInfer rejects block_tables (B, 1, 2176) against sparse_mla_top_k=2048. Mirror the generic flashinfer_mla_sparse backend: derive sparse_mla_top_k/max_seq_len from the converted buffer width; the kernel treats it as page-table capacity and bounds the active length by the -1 padding.

No behavior change for DSv3.2/DSv4 (RoPE, kpool=1): the packed row write is identical when pe_dim == 64, and shape[1] == topk_tokens when there is no kpool tail.

Requires FlashInfer with GLM53_NOPE support plus the companion check-scoping fix flashinfer-ai/flashinfer#4947 — both ride the same post-flashinfer#4802 release boundary, so no additional wait.

Relation to #53969

#53969 made this model servable before the native GLM53_NOPE kernel existed, by zero-padding the rope lane on both the KV-write and query side and routing through the GLM_NSA kernel — its own comment notes the shim "can be removed if the kernels grow native pe_dim == 0 support". This PR is that native support: no padding writes, no wasted rope MMA (~11% of the QK dims), and the decode goes to the dedicated GLM53_NOPE kernels with per-H tuning. The effective-topk half is the same fix (credit to @hamiltongaianimd for surfacing the kpool width issue first); whichever PR lands second rebases.

Test Plan

• Hardware: 4×RTX PRO 6000 (SM120), TP4, GLM-5.3-Flash-NVFP4 (text-only, --language-model-only), max_model_len=65536, FULL_AND_PIECEWISE cudagraphs.
• Server boots, CUDA graph capture succeeds, completions are coherent.
• GSM8K strict-match: 0.9325 (1319 problems, concurrency 32, max_tokens 2048), consistent with the 0.973 reported for this model on GB200 in [KV Cache] GLM-5.3-Flash: use the generic packed KV layout; CircularBufferSpec tail with speculative ring slots #55219.

Test Result

Pass. Without these commits the same server crashes at startup (pe_dim must be 64 for fp8_ds_mla) and then at capture (expects sparse block_tables shape (512, 1, 2048), got (512, 1, 2176)).

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Signed-off-by: Zihua Wu zihuaw@nvidia.com

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📝 Summary

Summary by CodeRabbit

• New Features

  • Added support for NoPE models using the FP8 MLA cache format, including cache entries without positional-encoding data.

• Bug Fixes

  • Ensured reserved cache padding remains zeroed so attention consumers receive predictable values.
  • Improved sparse MLA attention handling by using the available physical top-k capacity, helping process requests with the correct sequence length and token limit.

Walkthrough

The MLA cache kernel now supports NoPE rows and zeroes reserved padding. The SM120 sparse backend derives top-k capacity from the physical buffer width. A kernel test validates the NoPE cache output.

Changes

MLA sparse path updates

Layer / File(s)	Summary
NoPE packed-cache support csrc/libtorch_stable/cache_kernels.cu, tests/kernels/attention/test_cache.py	NoPE rows zero bytes 528:656 instead of writing a RoPE tail. fp8_ds_mla accepts pe_dim == 0 and pe_dim == 64. The test validates FP8 data, scales, and zeroed padding.
Physical top-k capacity wiring vllm/v1/attention/backends/mla/flashinfer_mla_sparse_sm120.py	The SM120 sparse path derives capacity, max_seq_len, and sparse_mla_top_k from the physical top-k buffer width.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 8d098

This change enables NoPE sparse MLA cache rows and sizes sparse capacity from the physical top-k buffer. The cache layout and zeroed reserved bytes are covered by targeted testing, with no current merge-blocking risk identified.

Suggested reviewers: stefankoncarevic, lucaswilkinson

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	The PR supports the NoPE sparse backend, but #4947 requires moving sparse_mla_top_k_lens validation to non-sparse backends. No corresponding entry-point validation change appears in the provided chang…	Implement the #4947 validation-scope change in this PR, or provide explicit evidence that the target FlashInfer dependency already includes it and remove or reclassify the issue link as a dependency.
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 5 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the primary change: NoPE sparse MLA support for the FlashInfer SM120 backend.
Description check	✅ Passed	The description directly explains the two bug fixes, their scope, dependencies, and validation results.
Out of Scope Changes check	✅ Passed	The cache-layout fix, physical top-k capacity fix, and regression test all directly support NoPE sparse MLA execution. No unrelated changes are evident.

Full details: Linked Issues check

Explanation

The PR supports the NoPE sparse backend, but #4947 requires moving sparse_mla_top_k_lens validation to non-sparse backends. No corresponding entry-point validation change appears in the provided changes.

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Actionable comments posted: 1

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Inline comments:
In `@csrc/libtorch_stable/cache_kernels.cu`:
- Around line 545-546: Update the fp8_ds_mla gather logic around
cp_gather_and_upconvert_fp8_kv_cache so pe_dim == 0 does not copy or expose
stale cache bytes as k_pe in workspace[..., 512:576]. Add a NoPE-aware path that
leaves the tail ignored or safely initialized, and add a regression test using a
sentinel value to verify NoPE consumers cannot read stale tail data.

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @lucifer1004.

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