[bobboli]

📌 Description

Follow-up to #5532. flashinfer.moe_ep.NVLinkOneSidedAlltoAll now runs its own NVLink one-sided MoE all-to-all kernels, synced with TensorRT-LLM's (cpp/tensorrt_llm/kernels/moe/communication/ plus the later one-sided changes), instead of wrapping the deprecated flashinfer.comm.MoeAlltoAll. The MoEEpCommunication interface from #5532 is unchanged.

It also completes the deprecation started in #5532. No MoEEpCommunication backend builds on MoeAlltoAll or MnnvlMoe any more, so both classes are removed (breaking; see Removal and migration below). NVLinkOneSidedAlltoAll and CakeAlltoAll take over MoeAlltoAll's checkpoint_prepare / checkpoint_restore.

What the new kernels bring

• CFT counted writes. On Blackwell systems whose driver exports the CUDA logical-endpoint API, dispatch and combine can push payloads over the NVLink fabric with fabric.try_put.counted. The receiver then waits on hardware byte counters instead of per-round completion flags, and the CFT dispatch also fills the padding rows' expert ids itself. The logical endpoint is bound to the existing MNNVL workspace, so no extra memory is allocated.
• Up to 256 EP ranks (was 64); the active-rank mask grows to four words.
• Workspace laid out by phase. Round state, dispatch control and payload, and combine control, input and the CFT receive inbox are separate regions with fixed capacities, which every op validates.
• Routing and fan-out.
  • Invalid expert ids are skipped during routing.
  • When top_k > ep_size, dispatch and combine fan out over a compact per-rank destination list.
  • Receive counters are double-buffered by round parity; the CFT dispatch publishes them right after routing.
• Low-precision combine pairs FP8 conversions.
• Timeouts are chosen on the host per launch.

Python

• NVLinkOneSidedAlltoAll drives the new JIT module (moe_ep_nvlink_one_sided_alltoall) directly.
  • Workspace sharing: instances with the same layout share one workspace, which is freed when its last instance is destroyed. Instances on one workspace must run their rounds one after another; a dispatch while another instance's round awaits its combine raises.
  • CFT setup: endpoint creation and exchange run over the EP group's CommBackend, so no MPI is involved. Every rank agrees on CFT before the layout is fixed, and a failure on any rank falls back to the fence path on all ranks.
• NVLinkOneSidedConfig gains:
  • cft: None uses CFT automatically where supported, for steps up to cft_max_tokens_for_dispatch / cft_max_tokens_for_combine tokens (128 each); True uses it on every supported step; False never uses it;
  • timeout_sec: the bound on each wait for a peer;
  • enable_pdl: programmatic dependent launch, None enabling it on SM90+ as before (also on CakeAlltoAllConfig).
• NVLinkOneSidedAlltoAll.active_rank_mask(ranks) builds the 256-rank mask, and cft_enabled reports whether CFT is available to the instance.
• CakeAlltoAll is now a standalone MoEEpCommunication (it used to subclass NVLinkOneSidedAlltoAll). The generated Cake kernels implement the protocol of the functional moe_a2a_* ops, which it calls with backend="cake"; those ops are unchanged. CakeAlltoAll pins the combine region at the end of the workspace instead of using the offset each dispatch returns, which moves with the token count (the race fix: keep one-sided MoE combine workspace offset stable #6376 fixed in MoeAlltoAll), and follows the same one-round-per-workspace rule.
• benchmarks/moe_ep/core/comm/bench_moe_ep_comm.py gets --cft {auto,on,off}.
• The module is in the AOT build list for SM90+.

Checkpointing

NVLinkOneSidedAlltoAll and CakeAlltoAll gain checkpoint_prepare() and checkpoint_restore(comm_backend), with the semantics MoeAlltoAll had (callers such as vLLM used to reach them through the former .alltoall accessor):

• Prepare: releases the physical MNNVL memory and keeps the virtual addresses reserved.
• Restore: remaps the memory over a fresh CommBackend and re-initializes the workspace. CUDA graphs captured before the checkpoint replay correctly afterwards.
• Shared rules: both calls are collective, and repeated calls do nothing.
• CFT endpoints:
  • NVLinkOneSidedAlltoAll also releases its CFT logical endpoints before the memory and recreates them afterwards.
  • Launches bake the endpoint IDs into their kernel parameters, so the endpoints keep their ID block reserved across the checkpoint (moe_a2a_cft_release_endpoints) and are recreated under the same IDs.
  • Restore verifies the IDs and raises if they changed.
• CftLeManager fix: it now tracks imported peer endpoints explicitly. It used to treat ID 0 as unset, and ID blocks start at 0, so the import of rank 0's endpoint was never destroyed. The kernels' copy in TensorRT-LLM has the same bug.
• The shared MNNVL helpers are unmap_mnnvl_memory / remap_mnnvl_memory in nvlink_common.

Removal and migration

MoeAlltoAll and MnnvlMoe were deprecated in #5532, which ships in a release before this PR. No backend uses them after this PR:

• NVLinkOneSidedAlltoAll runs the new kernels above.
• CakeAlltoAll calls the functional moe_a2a_* ops with backend="cake". Instances with the same geometry share a workspace, which the last one to be destroyed frees, as NVLinkOneSidedAlltoAll does.
• NVLinkTwoSidedAlltoAll calls moe_prepare / moe_comm from flashinfer.comm.trtllm_alltoall directly. Its FIFO positions persist in the workspace, so, as with MnnvlMoe, the workspaces stay with the process and are shared by every instance of the EP group.

This PR removes both classes, the flashinfer.comm.MoeAlltoAll export and MoEAlltoallInfo. The functional ops and their kernels stay. Migration:

Removed	Use instead
flashinfer.comm.MoeAlltoAll	flashinfer.moe_ep.NVLinkOneSidedAlltoAll (newer kernels), flashinfer.moe_ep.CakeAlltoAll for backend="cake", or the moe_a2a_* ops for arbitrary payload lists
MoeAlltoAll.checkpoint_prepare / checkpoint_restore	the methods of the same names on NVLinkOneSidedAlltoAll / CakeAlltoAll
flashinfer.comm.trtllm_alltoall.MnnvlMoe	flashinfer.moe_ep.NVLinkTwoSidedAlltoAll, or moe_prepare / moe_comm

Tests and benchmarks that used the classes now use the replacements:

• Tests that drove the kernels through the classes use the functional ops, through tests/comm/moe_a2a_driver.py where they need per-round state. Class-only checks (method signatures, the workspace cache) are dropped.
• The moe_a2a_dispatch_combine routine of flashinfer_benchmark.py runs the functional ops.
• bench_cute_dsl_moe_distributed.py uses NVLinkOneSidedAlltoAll.

Files

• csrc/nv_internal/tensorrt_llm/kernels/moe/communication/: kernels, CFT logical-endpoint manager, CUDA 13.4 feature gate. The paths mirror TensorRT-LLM's.
• csrc/nv_internal/tensorrt_llm/thop/moe/communication/moeAlltoAllMeta.h: workspace layout indices.
• csrc/moe_ep_nvlink_one_sided_alltoall.cu: TVM-FFI ops.
• flashinfer/jit/comm.py: gen_moe_ep_nvlink_one_sided_module.
• flashinfer/moe_ep/backends/split/comm/{nvlink_one_sided,cake}/.

🔍 Related Issues

Follow-up to #5532 and #4784.

🚀 Pull Request Checklist

✅ Pre-commit Checks

• I have installed pre-commit by running pip install pre-commit (or used your preferred method).
• I have installed the hooks with pre-commit install.
• I have run the hooks manually and fixed any reported issues (run on the changed files).

🧪 Tests

• Tests have been added or updated as needed.
• All tests are passing (unittest, etc.).

Validated on B300 (CUDA 13.5, driver 620, where CFT counted writes are available):

• tests/moe_ep/test_nvlink_one_sided.py (new; pytest spawns one process per EP rank; also run by run_tests.sh comm): 44 passed. Ported from TensorRT-LLM's test_nvlink_one_sided.py: model-shaped round trips (MXFP8, blockwise FP8, NVFP4 and BF16 dispatch; top_k 3 to 16; EP4 and EP8; 1, 128 and 1024 tokens), BF16/FP8 combine with external or workspace input, a 4-rank round sequence with changing and zero token counts, a delayed rank and the CFT threshold (fence, CFT and automatic; eager and CUDA graph; PDL on and off), EPLB statistics and the workspace layout. The round sequence, combine, EPLB and model cases also run on Cake; with Cake's former moving combine offset the round sequence fails with mismatched outputs.
• The module also builds for FLASHINFER_CUDA_ARCH_LIST="9.0a 10.0a 10.3a".
• tests/moe_ep/test_moe_ep_communication_multirank.py on 4 and 8 GPUs (bash tests/moe_ep/run_tests.sh comm), 18 passed each. The one-sided cases now run as a fence variant and a CFT variant:
  • dispatch/combine against a local reference at top_k 4 and 8 (rank-compact fan-out);
  • FP8 combine together with EPLB statistics and an all-active rank mask;
  • split-layer CUDA-graph replay;
  • Cake and two-sided, now off the removed classes, still pass;
  • checkpoint/restore around a captured split-layer CUDA graph for the fence, CFT and Cake paths (4 and 8 GPUs, 3 passed each).
• tests/moe_ep/test_communication.py (host-only, stubbed kernels): 31 passed. New cases cover workspace sizing by dispatch format, CFT selection and probe failures, enable_pdl, payload plumbing and sanitization, the rank mask, config validation, Cake's pinned combine region, and rejecting interleaved rounds on a shared workspace.
• bash tests/moe_ep/run_tests.sh unit: 961 + 1 + 23 passed, 141 skipped.
• The functional ops and their tests in tests/comm, now without the classes:
  • test_trtllm_moe_alltoall.py and test_moe_a2a_recv_view_cache.py: 167 passed;
  • test_mnnvl_moe_alltoall.py and test_mnnvl_moe_alltoall_fused_adapter.py (mpirun -np 4): 41 passed, 1 skipped; the fused adapter's two-rank cycle (mpirun -np 2): 24 passed;
  • test_mnnvl_memory.py (mpirun -np 4): 2 passed.
• flashinfer_benchmark.py --routine moe_a2a_dispatch_combine --validate (plain, --quant_dtype nvfp4, --use_lora) validates on 4 ranks.
• bench_cute_dsl_moe_distributed.py --variants w4a16 runs on 8 GPUs.

Reviewer Notes

• FlashInfer-only features of the old one-sided kernels stay available through the moe_a2a_* ops and are not ported, because MoEEpCommunication does not expose them:
  • combine output quantization (MXFP8/MXFP4/NVFP4);
  • the phased NVFP4 dispatch for hidden ≤ 2048 at EP4/8/16 (perf(moe): phase NVFP4 dispatch payload copies at EP4/EP8/EP16 #4067).
• CFT limits:
  • only one workspace per process can bind the logical endpoints; others use the fence path;
  • payload rows must be multiples of 16 bytes, so top_k = 3 or 6 routing (12- or 24-byte id rows) dispatches over the fence path and only its combine can use CFT;
  • the kernels compile CFT in only with CUDA 13.4+ headers, and the feature is resolved from the driver at runtime.
• An instance with enable_rank_mask=True refuses CUDA-graph capture, because the mask would be frozen into the graph.

Follow-ups (separate PRs)

• Retire the legacy backend="trtllm" path of the moe_a2a_* ops (JIT module mnnvl_moe_alltoall, kernels in csrc/nv_internal/tensorrt_llm/kernels/communicationKernels/moeAlltoAllKernels.*). No moe_ep backend uses it after this PR. Deprecate it first, then remove it together with its tests (test_trtllm_moe_alltoall.py, test_mnnvl_moe_alltoall.py / moe_a2a_driver.py, test_moe_a2a_recv_view_cache.py; keep the parts that apply to Cake), and change the backend default. The FlashInfer-only features listed above (combine output quantization, phased NVFP4 dispatch) need a decision before that: port them or drop them. Until then, note that the module-level moe_a2a_dispatch returns a combine offset that moves with runtime_max_tokens_per_rank and the payload sizes (for both trtllm and cake); callers that drive the ops directly must keep the combine region fixed themselves, as CakeAlltoAll does.
• Retire benchmarks/routines/moe_comm.py (moe_a2a_dispatch_combine), which times the legacy path. Communication-only cases move to benchmarks/moe_ep/core/comm/bench_moe_ep_comm.py, which needs fp8_block_scale (DeepSeekFp8) and an --extra_payload_bytes option (covers --use_lora). Dispatch + MoE + combine cases (--real_math) move to benchmarks/moe_ep/bench_moe_ep.py, which needs the NVLink backends (nvlink_one_sided, cake, nvlink_two_sided). Optionally register bench_moe_ep_comm.py as a flashinfer_benchmark routine.
• Move the Cake MoE all-to-all sources out of nv_internal (e.g. into csrc/cake_moe_alltoall/), and remove the per-phase files in communicationKernels/ that no JIT spec compiles (moeAlltoAllDispatch.cu, moeAlltoAllPrepareDispatch.cu, moeAlltoAllStageCombine.cu, moeAlltoAllPublishCombine.cu, moeAlltoAllCombine.cu, moeAlltoAllQuantizeCombine.cu, moeAlltoAllSanitize.cu).
• MNNVL handle selection (see fix: prefer usable FABRIC handles with single-host POSIX fallback #5765): pick the handle type from the first real allocation (create/export, vote, import, vote, then map), fall back to POSIX on a single host, and share one cached decision between MnnvlMemory and SymmDeviceMemory. CFT should then follow the handle type the workspace actually uses.

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Added NVLink one-sided expert-parallel communication with optional counted-write transport, active-rank masking, low-precision combine, and checkpoint/restore support.
  • Cake communication now provides dispatch, combine, workspace management, and checkpoint/restore.
  • Added options for counted-write selection, token limits, and communication timeouts.
• Improvements
  • Updated benchmarks and communication paths to use functional dispatch and combine operations.
  • Added access to distributed allocation handles.
• API Changes
  • Removed the legacy MoeAlltoAll and MnnvlMoe interfaces.

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• flashinfer/moe_ep/backends/split/comm/nvlink_one_sided/communication.py
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📝 Walkthrough

The change adds NVLink one-sided MoE all-to-all operations, updates Cake and NVLink backends, and moves benchmarks and tests to functional communication APIs. It removes legacy MoeAlltoAll and MnnvlMoe wrappers and updates related documentation.

Changes: MoE all-to-all communication

Layer / File(s)	Summary
One-sided kernel contracts and operations csrc/moe_ep_nvlink_one_sided_alltoall.cu, csrc/nv_internal/tensorrt_llm/kernels/moe/communication/*, csrc/nv_internal/tensorrt_llm/thop/moe/communication/*, csrc/nv_internal/include/tensorrt_llm/common/tllmDataType.h, flashinfer/jit/...	Adds workspace layout and initialization operations, CFT endpoint lifecycle operations, and dispatch, combine, and sanitization operations. Adds kernel parameter declarations and a JIT module builder.
NVLink backend and CFT lifecycle flashinfer/moe_ep/backends/split/comm/nvlink_one_sided/*, flashinfer/moe_ep/backends/split/comm/nvlink_common.py, flashinfer/comm/mnnvl.py, benchmarks/moe_ep/core/comm/bench_moe_ep_comm.py, tests/moe_ep/test_communication.py, tests/moe_ep/test_moe_ep_communication_multirank.py	The backend manages shared workspace state and selects fence or CFT transport based on configuration and eligibility. It adds active-rank mask and checkpoint handling. Tests cover dispatch, combine, CFT selection, and checkpoint behavior; the benchmark adds a --cft option.
Cake and NVLink two-sided backends flashinfer/moe_ep/backends/split/comm/cake/*, flashinfer/moe_ep/backends/split/comm/nvlink_two_sided/communication.py, tests/moe_ep/test_communication.py	Cake now directly implements MoEEpCommunication with its own workspace, dispatch, combine, and checkpoint logic. NVLink two-sided communication uses moe_prepare and moe_comm with cached workspaces.
Functional API migration and legacy wrapper removal flashinfer/comm/*, benchmarks/bench_cute_dsl_moe_distributed.py, benchmarks/routines/moe_comm.py, tests/comm/*, docs/api/comm.rst, docs/design_docs/moe_ep_architecture.md	Benchmarks and tests use functional MoE all-to-all operations or backend communication classes. Legacy wrapper APIs and the MoeAlltoAll re-export are removed. Documentation describes the updated backends and checkpoint methods.

Priority: ➖ Normal

Estimated code review effort: 5 (Critical) | ~100 minutes

Change: Feature

Merge Risk: 🟡 Moderate · up to e0320

Two open concerns could still affect MoE all-to-all startup. A peer's first Cake dispatch may race workspace initialization. NVLink one-sided construction may also fail, or ranks may hang, on platforms where the fabric probe raises an error. Confirm or fix both before merging.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 43.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 180 functions across 27 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title clearly and concisely identifies the main changes: synchronizing NVLinkOneSidedAlltoAll with TensorRT-LLM kernels and removing deprecated APIs.
Description check		The description is complete and directly aligned with the template. It explains the changes, migration path, related issues, testing, checklist status, reviewer notes, and follow-ups.

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🚨 POTENTIAL BREAKING PUBLIC API CHANGE DETECTED 🚨

Caution

THIS PR APPEARS TO BREAK THE PUBLIC API. AUTHORS AND REVIEWERS: DO NOT MISS THIS.

This is an advisory warning and does not gate merging. Confirm compatibility and provide a deprecation or migration path, or track the fix in a follow-up PR.

6 public API finding(s):

• flashinfer/comm/trtllm_moe_alltoall.py:1237 — Public API flashinfer.comm.trtllm_moe_alltoall.MoeAlltoAll.checkpoint_prepare was removed; update deprecation and API documentation.
• flashinfer/comm/trtllm_moe_alltoall.py:1257 — Public API flashinfer.comm.trtllm_moe_alltoall.MoeAlltoAll.checkpoint_restore was removed; update deprecation and API documentation.
• flashinfer/comm/trtllm_moe_alltoall.py:1479 — Public API flashinfer.comm.trtllm_moe_alltoall.MoeAlltoAll.combine was removed; update deprecation and API documentation.
• flashinfer/comm/trtllm_moe_alltoall.py:1374 — Public API flashinfer.comm.trtllm_moe_alltoall.MoeAlltoAll.dispatch was removed; update deprecation and API documentation.
• flashinfer/comm/trtllm_moe_alltoall.py:1577 — Public API flashinfer.comm.trtllm_moe_alltoall.MoeAlltoAll.get_combine_payload_tensor_in_workspace was removed; update deprecation and API documentation.
• flashinfer/comm/trtllm_moe_alltoall.py:1009 — Public API flashinfer.comm.trtllm_moe_alltoall.MoeAlltoAll.get_moe_workspace_size_per_rank was removed; update deprecation and API documentation.

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Inline comments:
Review comments at @flashinfer/jit/comm.py:
- Around line 379-409: Add gen_moe_ep_nvlink_one_sided_module to the appropriate
AOT build list in flashinfer/aot.py alongside the existing MoE all-to-all
generators, so NVLinkOneSidedAlltoAll is available when JIT is disabled.

Review comments at @flashinfer/moe_ep/backends/split/comm/cake/communication.py:
- Around line 132-165: Track the in-flight round owner on the shared state
created by `_acquire_workspace`; in `dispatch`, reject a round if another
instance owns that workspace, then mark the current instance as owner. Clear
ownership when the round completes in `combine` and when the instance is
destroyed, preserving the per-instance `self._round` guard.

Review comments at
@flashinfer/moe_ep/backends/split/comm/nvlink_one_sided/communication.py:
- Around line 85-93: Update _cft_unsupported_reason to catch exceptions from
is_mnnvl_fabric_supported and the CUDA attribute probe beyond AttributeError,
returning an unsupported-reason string so construction can fall back to the
fence transport instead of failing.

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• benchmarks/bench_cute_dsl_moe_distributed.py
• benchmarks/moe_ep/core/comm/bench_moe_ep_comm.py
• benchmarks/routines/moe_comm.py
• csrc/moe_ep_nvlink_one_sided_alltoall.cu
• csrc/nv_internal/include/tensorrt_llm/common/tllmDataType.h
• csrc/nv_internal/tensorrt_llm/kernels/moe/communication/.clang-format
• csrc/nv_internal/tensorrt_llm/kernels/moe/communication/moeAlltoAllCftManager.h
• csrc/nv_internal/tensorrt_llm/kernels/moe/communication/moeAlltoAllCftSupport.h
• csrc/nv_internal/tensorrt_llm/kernels/moe/communication/moeAlltoAllKernels.cu
• csrc/nv_internal/tensorrt_llm/kernels/moe/communication/moeAlltoAllKernels.h
• csrc/nv_internal/tensorrt_llm/thop/moe/communication/.clang-format
• csrc/nv_internal/tensorrt_llm/thop/moe/communication/moeAlltoAllMeta.h
• docs/api/comm.rst
• docs/design_docs/moe_ep_architecture.md
• flashinfer/comm/__init__.py
• flashinfer/comm/mnnvl.py
• flashinfer/comm/trtllm_alltoall.py
• flashinfer/comm/trtllm_moe_alltoall.py
• flashinfer/jit/__init__.py
• flashinfer/jit/comm.py
• flashinfer/moe_ep/backends/split/comm/cake/communication.py
• flashinfer/moe_ep/backends/split/comm/cake/config.py
• flashinfer/moe_ep/backends/split/comm/nvlink_common.py
• flashinfer/moe_ep/backends/split/comm/nvlink_one_sided/communication.py
• flashinfer/moe_ep/backends/split/comm/nvlink_one_sided/config.py
• flashinfer/moe_ep/backends/split/comm/nvlink_one_sided/kernels.py
• flashinfer/moe_ep/backends/split/comm/nvlink_two_sided/communication.py
• tests/comm/moe_a2a_driver.py
• tests/comm/test_mnnvl_memory.py
• tests/comm/test_mnnvl_moe_alltoall.py
• tests/comm/test_mnnvl_moe_alltoall_fused_adapter.py
• tests/comm/test_trtllm_moe_alltoall.py
• tests/moe_ep/test_communication.py
• tests/moe_ep/test_moe_ep_communication_multirank.py

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• flashinfer/comm/init.py
• flashinfer/comm/trtllm_alltoall.py

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[bobboli]

test_moe_a2a_workspace.py only tested internals of MoeAlltoAll (_reserve_combine_region / _check_dispatch_region), which this PR removes, so it cannot be kept as is. TensorRT-LLM did the same in #19610: it dropped that unit test and kept a multi-rank regression in test_nvlink_one_sided.py. This PR now ports that file as tests/moe_ep/test_nvlink_one_sided.py (a56ba5c, 75c16f2), which covers what the deleted test protected:

• test_workspace_layout: dispatch, combine and CFT receive regions are disjoint, aligned and sized at allocation;
• a 4-rank round sequence with changing and zero token counts, a delayed rank in every round, the CFT threshold, fence/CFT/auto, eager and CUDA graph, PDL on and off, plus model-shaped cases, FP8/workspace combine and EPLB;
• the same round sequence and combine cases on Cake, whose combine region is now pinned as well (it fails with the old moving offset).

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