nvfp4-megamoe-kernel/README.md

nvfp4-megamoe-kernel

Native NVFP4 block-scaled MoE kernel for DeepSeek-V4-Pro on NVIDIA Blackwell (SM100).

Replaces the broken fp8_nvfp4_mega_moe kernel from DeepGEMM with a working CUTLASS-based implementation that emits real SM100_MMA_MXF4_SS tensor core instructions.

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Architecture

DeepSeek-V4-Pro is a 256-expert MoE model with expert parallelism across 8 ranks (B200 GPUs). Each rank handles 32 experts. For each token, the router picks the top-6 experts.

The MoE Forward Pass

Input hidden states (BF16)
        │
        ▼
┌─────────────────┐
│  Shared Experts │  ← vLLM native FlashInfer CUTLASS NVFP4 path
│  (gate + up →  │     (not our kernel)
│   SiLU * up →   │
│   down)         │
└─────────────────┘
        │
        ▼
  Staging Kernel (vLLM built-in)
  BF16 → packed E2M1 (int8) + UE4M3 block-16 scales (uint32)
  Writes to SymmBuffer.x / SymmBuffer.x_sf
        │
        ▼
  Router (vLLM built-in)
  Writes topk_ids / topk_weights to SymmBuffer
        │
        ▼
┌─────────────────────────────────────────┐
│          nvfp4_mega_moe_full            │  ← nvfp4_mega_moe.py
│                                         │
│  1. Read staged activation from buffer  │
│  2. L1 GEMM: gate_up_proj              │  ← CUTLASS NVFP4 block-scaled
│     E2M1 × E2M1 + UE4M3 scales         │    SM100_MMA_MXF4_SS PTX
│     → BF16 output (6144-wide)          │
│  3. SiLU(gate) * up  (activation)      │
│  4. stage_activation: BF16 → FP4       │  ← proper E2M1 quantization
│ 5. L2 GEMM: down_proj                 │  ← CUTLASS NVFP4 block-scaled
│     E2M1 × E2M1 + UE4M3 scales         │    SM100_MMA_MXF4_SS PTX
│     → BF16 output (7168-wide)          │
│  6. Write to output tensor              │
└─────────────────────────────────────────┘

vLLM Startup Sequence (how our code plugs in)

1. vLLM engine init
   └─ ModelOptNvFp4Config selected (NVFP4 quantization scheme)
   └─ FlashInferCutlassNvFp4LinearKernel for linear layers

2. Model construction
   └─ DeepseekV4ForCausalLM → DeepseekV4MoE → DeepseekV4DecoderLayer
       Each layer has: attention + MoE block
       MoE block has: shared experts + 256 routed experts

3. Weight loading
   └─ 95 safetensor shards loaded
   └─ weight, weight_scale, weight_scale_2 loaded per linear

4. process_weights_after_loading  ← THIS IS WHERE WE HOOK IN
   └─ ModelOptNvFp4LinearMethod swizzles/pads weights for CUTLASS
   └─ finalize_mega_moe_weights()
       └─ weight_transform.py: transform_nvfp4_weights_for_mega_moe()
           • Folds weight_scale_2 (global scale) into weight_scale (block scale)
           • UE4M3 block-16 scales: 4 values packed per uint32
           • Returns ((l1_w, l1_sf), (l2_w, l2_sf)) per rank

5. SymmBuffer allocation
   └─ symm_buffer.py: get_symm_buffer_for_nvfp4_mega_moe()
       • Pre-allocates GPU buffers for:
         - x: int8 packed E2M1 activations
         - x_sf: uint32 packed UE4M3 activation scales
         - topk_idx: int32 expert indices
         - topk_weights: float32 routing weights
         - buffer: BF16 all-reduce buffer

6. Profile run (warmup)
   └─ First forward pass to allocate KV cache, etc.
   └─ This is where the CUTLASS GEMM first executes

7. Ready to serve

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File Map

nvfp4_megamoe_kernel/
├── __init__.py              # Public API exports
├── nvfp4_mega_moe.py       # Main kernel: nvfp4_mega_moe_full, nvfp4_mega_moe_l1/l2, stage_activation
├── weight_transform.py     # Weight prep: fold global scale, pack UE4M3
├── symm_buffer.py          # GPU buffer allocation for MoE dispatch
│
└── cutlass_nvfp4_gemm/     # CUTLASS CUDA extension (the actual hardware kernel)
    ├── cutlass_nvfp4_gemm.cu   # CUDA: CUTLASS GEMM + SF remap kernel
    ├── pytorch_binding.cpp     # PyTorch C++ binding (_C.forward)
    ├── kernel.py               # Python: cutlass_grouped_nvfp4_gemm (per-expert loop)
    ├── sf_layout.py            # CUTLASS SF layout reference docs
    ├── setup.py                # Build config (nvcc, CUTLASS include paths)
    ├── build.sh                # Build script
    ├── test_gemm.py            # Standalone test
    └── README.md

What each file does (in call order)

File	When it runs	What it does
weight_transform.py	Once at startup (weight loading)	Takes raw NVFP4 checkpoint weights, folds global scales into block scales, packs UE4M3 into uint32. Output: ((l1_w, l1_sf), (l2_w, l2_sf))
symm_buffer.py	Once at startup (buffer alloc)	Pre-allocates GPU tensors for activations, scales, routing data, and all-reduce. These persist across forward passes.
nvfp4_mega_moe.py	Every forward pass	Orchestrates the MoE: reads from symm buffer → L1 GEMM → activation → re-quantize → L2 GEMM → output. Contains stage_activation (BF16→FP4 quantize for L1→L2) and unpack_ue4m3_u32 (uint32 packed scales → float8).
cutlass_nvfp4_gemm/kernel.py	Every forward pass (called by nvfp4_mega_moe)	Per-expert loop: gather tokens for each expert, call CUTLASS GEMM, scatter results with routing weights.
cutlass_nvfp4_gemm/cutlass_nvfp4_gemm.cu	Every forward pass (CUDA kernel)	The actual CUTLASS kernel: native NVFP4 block-scaled GEMM + GPU-side scale factor remap (row-major → CUTLASS interleaved layout).
cutlass_nvfp4_gemm/sf_layout.py	Reference only	Documents the CUTLASS SfAtom layout. Not used at runtime (remap is in CUDA).

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Data Formats

Weights

• Packed E2M1 (int8): 2 FP4 values per byte. Shape: (E_per_rank, N, K//2), K-major layout.
• UE4M3 block scales (float8_e4m3fn): 1 scale per 16 FP4 values (group_size=16). Shape: (E_per_rank, N, K//16).

Activations (after staging kernel)

• Packed E2M1 (int8): Shape: (num_tokens, K//2).
• UE4M3 scales (uint32): 4 UE4M3 values packed per uint32. Shape: (num_tokens, K//64).

GEMM dimensions (DeepSeek-V4-Pro)

• L1 (gate_up_proj): M×6144×7168 (per expert)
• L2 (down_proj): M×7168×3072 (per expert)
• 48 experts per rank (256 total / 8 ranks), top-6 routing

---

CUTLASS Scale Factor Remap

CUTLASS's Sm1xxBlockScaledConfig expects scale factors in a specific interleaved layout, not simple row-major. The SfAtom is:

Atom Shape:  Shape<Shape<32, 4>, Shape<16, 4>>
Atom Stride: Stride<Stride<16, 4>, Stride<0, 1>>
Tiling:      Step<_2, _1>  (M tiled with step 2, K with step 1)

Our source data is row-major (M, K_sf) where K_sf = K / 16. The remap kernel (remap_sf_to_cutlass_kernel in cutlass_nvfp4_gemm.cu) converts from row-major to CUTLASS's interleaved layout.

How the remap works

The kernel iterates over CUTLASS destination indices, uses cute::idx2crd to get the hierarchical coordinate, then cute::flatten to get a flat tuple of 8 sub-indices. From those, we extract logical (m, k_sf) and read from the row-major source.

Flattened coordinate decomposition (flat_rank=8)

From the SfAtom layout with Step<_2, _1> tiling, flatten(idx2crd(idx, ...)) produces 8 values:

f0 = inner_m  (0..31)  — varies fastest within M atom
f1 = sub_m    (0..3)   — second M sub-coordinate
f2 = tile_m   (0..)    — M tile index
f3 = step_m stride     — degenerate (always = sfa_size, not a coordinate)
f4 = sub_k    (0..3)   — K sub-coordinate within atom
f5 = tile_k   (0..)    — K tile index
f6 = 0                  — unused
f7 = 0                  — unused

Empirical coordinate dump (MN=8192, K_sf=448, T = sfa_size = 58720256)

idx	f0	f1	f2	f3	f4	f5	f6	f7
0	0	0	0	T	0	0	0	0
1	0	0	0	T	1	0	0	0
4	0	1	0	T	0	0	0	0
16	1	0	0	T	0	0	0	0
511	31	3	0	T	3	0	0	0
512	0	0	0	T	0	1	0	0
1024	0	0	0	T	0	2	0	0
2048	0	0	0	T	0	4	0	0
4096	0	0	0	T	0	8	0	0
8192	0	0	0	T	0	16	0	0
65536	0	0	1	T	0	16	0	0
131072	0	0	2	T	0	32	0	0

Extraction formula

CuTe uses "first sub varies fastest" for Shape<32, 4>:

m    = f0 + f1 * 32 + f2 * 128;
k_sf = f4 + f5 * 4;

This was verified with 6 independent probes:

Probe	Source	Expected	Result
SFA[1, 0] = 2.0	row 1 changes	✅ only row 1	Confirms f0 term
SFA[32, 0] = 2.0	row 32 changes	✅ only row 32	Confirms f132, rules out f04+f1
SFA[128, 0] = 2.0	row 128 changes	✅ only row 128	Confirms f2*128
SFA[0, 1] = 2.0	row 0 changes (k=1)	✅ only row 0	Confirms f4 term
SFA[0, 4] = 2.0	row 0 changes (k=4)	✅ only row 0	Confirms f5*4 term
SFA[0, 100] = 2.0	row 0 changes (k=100)	✅ only row 0	Confirms tile-overflow range

Why the previous remap was broken

The previous code used cute::get<0>(flat) and cute::get<1>(flat) to extract (m, k). Since flatten produces (inner_m, sub_m, tile_m, ...) in order, get<0> and get<1> are both M sub-indices — they carry no K information. This caused only k_group=0 to work; all other K-groups were silently mapped to the wrong source offset.

Additionally, the dest buffer must be zero-initialized before remap because CUTLASS pads to tile boundaries (128 × 64), making the dest buffer larger than M * K_sf. Unmapped padding slots reading garbage caused sporadic wrong results.

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Bugs Found & Fixed

1. unpack_ue4m3_u32: value cast vs bit reinterpret

File: nvfp4_mega_moe.py
Bug: (x_u32 & 0xFF).to(torch.int32).to(torch.float8_e4m3fn) converts integer 63 → float8(63.0).
Fix: (x_u32 & 0xFF).to(torch.uint8).view(torch.float8_e4m3fn) reinterprets bit pattern 0x3F → float8(~0.984).
Also: uint32 lacks CUDA bitwise ops — cast to int32 first.
Impact: Corrupted every activation scale fed to the L1 GEMM. Weight scales were fine (already float8 from weight_transform). "Structured garbage" recipe.

2. stage_activation: three independent bugs

File: nvfp4_moe.py
Bug A: clamp(0, 15) zeroed every negative value. E2M1 is sign-magnitude 4-bit (bit3=sign, bits2:0=mag).
Bug B: Stored block_max but divided by block_max/6.0 → stored scale was 6× too large.
Bug C: Uniform 0.5 step doesn't match E2M1 values {0, ±0.5, ±1, ±1.5, ±2, ±3, ±4, ±6} — non-uniform above ±2.
Fix: Rewrote with proper nearest-neighbor E2M1 quantization.
Impact: Half the L1→L2 activation was zeroed, 6× scale mismatch, quantization noise on top.

3. _fold_global_scale: logical_widths branch

File: weight_transform.py
Bug: logical_widths=[3072, 3072] caused the function to apply expert 0's scale to gate half and expert 1's scale to up half of ALL experts. All other experts' global scales were discarded.
Fix: Removed the logical_widths branch entirely. The else branch correctly broadcasts each expert's own (E, 1) global scale across (E, N, K//16).

4. L1 weight interleave removed

File: weight_transform.py
Bug: _interleave_l1_weights assumed gate/up were pre-interleaved in groups of 16 and that the kernel used 2CTA UMMA layout. vLLM uses plain concat [gate; up] along the output dim, and our CUTLASS kernel uses ClusterShape<1, 1, 1>.
Fix: Removed entirely. l1_weight_out = l1_weight.contiguous().

5. SF remap: idx2crd+flatten coordinate extraction

File: cutlass_nvfp4_gemm.cu
Bug: cute::flatten(coord) produces 8 sub-indices (flat_rank=8). get<0> and get<1> are both M sub-indices (inner_m, sub_m), carrying zero K information. Only k_group=0 worked; all other K-groups were silently wrong.
Fix: Correct extraction: m = f0 + f1*32 + f2*128, k_sf = f4 + f5*4. Zero-init dest buffer before remap.
Diagnostic trail: Constant-scale test (all SF=1.0) → cosine 1.0 proved FP4 path was correct. Real scales → cosine 0.83 proved SF remap was broken. Single-element probes (SFA[0,0] vs SFA[0,3]) proved only k_group=0 worked. Printf dump of flat coordinates at specific indices revealed flat_rank=8 and the correct extraction formula.

Diagnostic: constant-scale test (smoking gun for SF bugs)

When all scale factors are set to UE4M3(1.0):

• Cosine = 1.0000, MSE = 0.19 (expected FP4 quantization noise)

With real (variable) scale factors and the broken remap:

• Cosine = 0.83 → scales are misaligned, not fundamentally broken

After the fix with correct coordinate extraction:

• Cosine = 1.0000, MSE = 0.0 → perfect match with dequantized reference

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Build & Deploy (B200)

# On B200 host — CUTLASS must be cloned and mounted
cd /root/nvidia-meeting/deepseek-v4-quant/

# Rebuild container (CUTLASS is host-mounted at /root/cutlass)
KERNEL_CACHE_BUSTER=$(date +%s) docker compose build --no-cache
docker compose up -d

The CUTLASS extension builds inside the container during pip install of the nvfp4-megamoe-kernel package. It needs:

• CUDA 13.0 toolkit (in the vllm/vllm-openai:nightly image)
• CUTLASS headers at /root/cutlass/include/
• CCCL headers at /usr/local/cuda-13.0/targets/x86_64-linux/include/cccl/
• Device with SM100 compute capability (B200)

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Known Issues

1. MoE dispatch is slow — cutlass_grouped_nvfp4_gemm uses a Python loop over 48 experts with per-token scatter/gather. Needs a proper grouped GEMM or at least CUDA-side dispatch.

2. stage_activation is Python — Re-quantization from L1 BF16 output to FP4 for L2 input runs in PyTorch. Should use the Triton staging kernel for speed and consistency with vLLM's built-in staging.

3. SF remap allocates every call — cudaMemset + remap kernel runs per GEMM invocation. Could pre-compute the CUTLASS-layout buffer once during weight transform.

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Environment Variables

Variable	Default	Description
MEGA_MOE_STATIC	0	Set to 1 to skip MoE kernel entirely (return zeros)
MEGA_MOE_DEBUG	0	Set to 1 for verbose logging
SKIP_ATTENTION	0	Skip attention layers (debug)

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Repos

• Kernel: sweetapi.com/biondizzle/nvfp4-megamoe-kernel (branch: master)
• Deployment: sweetapi.com/biondizzle/deepseek-v4-quant (branch: modelopt-nvfp4)
• Local: ~/dev/nvfp4-mojo/, ~/dev/deepseek-v4-quant/