NVFP4 MegaMoE Kernel

Native NVFP4 inference stack for DeepSeek-V4 on NVIDIA Blackwell (SM100). CuTeDSL kernels for the entire model — MoE experts, shared experts, attention projections — running in native NVFP4 with zero dequantization overhead.

⚠️ THE #1 RULE

WE OWN ALL OUR KERNELS. WE DO NOT PATCH vLLM.

vLLM's internal kernels (FlashMLA, fp8_ds_mla, fused compressor, Triton indexer) are deeply coupled. You cannot swap one piece and expect the rest to work. We build our own CuTeDSL kernels, test standalone, then wire into vLLM as an attention backend.

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Repository Layout

This repo (nvfp4-megamoe-kernel): The kernel library — CuTeDSL kernels, bridge layer, standalone tests.

vLLM fork (vllm-deepseekv4-nvfp4): The vLLM integration — model definition, weight loading, attention backend. Lives at /root/dsv4-nvfp4-workspace/vllm on the B200.

Workspace (/root/dsv4-nvfp4-workspace):

• kernel/ — clone of this repo
• vllm/ — clone of the vLLM fork

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Kernel Status

✅ CuTeDSL NVFP4 Grouped GEMM (the building block)

ScaledGroupedGemmKernel in cutedsl/kernel/moe/torch_scaled_grouped_mm.py:

• 2D×3D scenario: A(M,K) × B(E,K,N) → C(M,N)
• Block-scaled: per-16-element FP8 scales on both A and B sides
• Global scales (per-expert) for full dynamic range
• Persistent scheduler, TMA pipelining, SMEM swizzle
• CUDAGraph-safe (workspace pre-allocated, no runtime allocations)

✅ Fused SwiGLU GEMM (L1 gate+up with SwiGLU in registers)

FusedSwiGLUScaledGroupedGemmKernel in cutedsl/kernel/moe/fused_swiglu_grouped_mm.py:

• Extends the base GEMM with an in-epilogue SwiGLU
• Weight interleave: interleave_l1_weights() interleaves gate/up at granularity 8 BF16
• epi_tile=(128, 8): each 8-wide subtile is pure gate or pure up
• Subtile-level pairing: even subtiles = gate (SiLU in FP32, save to register buffer), odd subtiles = up (load silu(gate) from buffer, compute silu(gate)*up)
• Output: BF16 with interleaved [silu(gate), silu(gate)*up] at granularity 8
• Cosine 0.988 vs BF16 reference (full MoE pipeline)

✅ Custom CUDA De-interleave + NVFP4 Quantize

cutedsl/kernels/deinterleave_quantize.cu:

• Single GPU kernel: reads fused L1 BF16 output, extracts SwiGLU from odd 8-col groups, quantizes to NVFP4
• Replaces the Python deinterleave_l1_weights() + quantize_activation_nvfp4() path
• 4.3x faster (0.043ms vs 0.184ms for 128 tokens)
• 99.97% cosine match with Python reference, 99.7% FP4 byte match
• Saves ~8.5ms over 60 MoE layers

✅ NVFP4 Linear (cutedsl/nvfp4_linear.py)

CuTeDSLNvfp4Linear — single-expert NVFP4 GEMM for shared experts and attention projections.

✅ GPU-Native SWA Decode Attention (CuTeDSL)

cutedsl/native_swa_decode.py — BlackwellSWADecodeKernel:

• CTA mapping: 1 CTA per (decode_token, q_head_group) — 8 groups × T tokens
• Q loaded into registers, KV streamed in 16-token tiles through smem
• Online softmax (max/exp/rescale/sum) across tiles
• Pre-dequantized bf16 KV (fp8 dequant done on host, fused dequant is future work)
• Cosine 0.9999+ vs PyTorch batched SDPA reference

✅ GPU-Native Sparse + SWA Decode Attention (CuTeDSL)

cutedsl/native_sparse_decode.py — BlackwellSparseDecodeKernel:

• Same CTA mapping as SWA kernel
• Concatenated SWA + compressed KV in a single attention pass
• Sink weight merge applied on host side
• Cosine 0.9999+ vs combined SDPA reference
• Supports both CSA (cr=4) and HCA (cr=128) layers

✅ Blackwell Attention (standalone tests)

• cutedsl/blackwell_attention.py — KV cache write/read, full attention pipeline
• cutedsl/csa_attention.py — CSA (cr=4) and HCA (cr=128) sparse attention
• All standalone tests pass: KV cache (0.9997), CSA/HCA, prefill+decode (0.9998)

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Bridge Layer (cutedsl/bridge.py)

Quantization, layout, kernel launch utilities:

Function	Purpose
quantize_to_nvfp4()	BF16 → NVFP4 with global scale
quantize_activation_nvfp4()	CUDAGraph-safe quantize (pre-computed gs)
quantize_weight_to_nvfp4()	Weight quantization along K dim
interleave_l1_weights()	Gate/up interleave at granularity 8 BF16
deinterleave_l1_weights()	Reverse the interleave
deinterleave_quantize_nvfp4_cuda()	Custom CUDA: de-interleave + quantize in one kernel
make_b_k_major()	B tensor stride conversion
assemble_scales_2d_side() / assemble_scales_3d_side()	Scale assembly + swizzle
warmup_compilation()	Eager JIT compilation (base GEMM)
warmup_fused_swiglu_compilation()	Eager JIT compilation (fused SwiGLU)
run_nvfp4_grouped_gemm()	Base GEMM entry point
run_fused_swiglu_grouped_gemm()	Fused SwiGLU GEMM entry point

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MoE Pipeline

Non-Fused Path

CuTeDSLMoERunner / run_nvfp4_moe():

1. Quantize input BF16 → NVFP4 (pre-computed gs)
2. L1 GEMM: NVFP4 × NVFP4 → BF16 (gate+up interleaved)
3. De-interleave, split gate/up
4. SiLU(gate) * up → BF16 (PyTorch)
5. Re-quantize BF16 → NVFP4
6. L2 GEMM: NVFP4 × NVFP4 → BF16 (down_proj)
7. Scatter with routing weights

Fused Path

run_nvfp4_moe_fused() / CuTeDSLMoERunner(fused_swiglu=True):

1. Quantize input BF16 → NVFP4 (pre-computed gs)
2. Fused L1 GEMM + SwiGLU in kernel registers → BF16 TMA store
3. Custom CUDA kernel: de-interleave + NVFP4 quantize (0.043ms)
4. L2 GEMM: NVFP4 × NVFP4 → BF16 (down_proj)
5. Scatter with routing weights

Both paths: cosine 0.988 vs BF16 reference. Fused path is marginally more accurate (FP32 SiLU in registers vs PyTorch BF16 SiLU).

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Correctness Bugs Fixed (May 20, 2026)

Bug	Issue	Fix
1	_needs_token_refill myth — cute.compile doesn't corrupt GPU memory	Removed hack, pre-allocated workspace per cache entry
2	Dequantize→requantize supposedly lossy	Verified 100% byte-identical round-trip. Deprecated prepare_weights_from_dequantized
3	clamp(min=1e-8) on zero blocks gives nonzero FP8 scale	Detect zero blocks, force FP8 scale to exact 0
4	Underflow blocks (amax < 6×2⁻⁹) get nonzero FP4 from div-by-tiny-number	Detect underflow blocks, zero x_norm before division
5	Expert counting materializes 18M bool tensor	torch.bincount replaces O(n×E) comparison

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Fused SwiGLU — How It Works

The Problem

The L1 GEMM produces (M, 2×intermediate) BF16 output with gate and up columns side by side. SwiGLU needs silu(gate)*up, producing (M, intermediate). In the unfused path, this requires:

• ~580MB BF16 write to GMEM (L1 output)
• ~290MB BF16 read back (for gate/up split + SiLU)
• 3 kernel launches + 12 quantize ops

The Solution: Granularity-8 Weight Interleave + Subtile Pairing

Key insight: With interleave_l1_weights(), gate and up weight columns are interleaved at granularity 8 BF16. In the GEMM output, every 8 BF16 columns alternate: [gate₀-₇, up₀-₇, gate₈-₁₅, up₈-₁₅, ...].

With epi_tile_n=8, each epilogue subtile covers exactly 8 BF16 N-columns. So each subtile is pure gate or pure up — no mixing. Even subtile indices = gate, odd = up.

The epilogue loop processes gate/up pairs:

for subtile_idx in range(subtile_cnt):
    acc_vec = load_accumulator(subtile_idx)
    acc_vec_bf16 = acc_vec.to(bf16)  # init before dynamic if

    if even (gate):
        silu_result = silu(acc_vec)    # FP32 math
        silu_gate_buf = silu_result    # save to register buffer
        acc_vec_bf16 = silu_result

    if odd (up):
        gate_vals = silu_gate_buf      # from previous iteration
        acc_vec_bf16 = gate_vals * acc_vec  # SwiGLU

    store_to_smem(acc_vec_bf16)
    tma_store_to_gmem()

Both branches produce acc_vec_bf16 of the same BF16 type. No runtime conditional affects tensor structure. The silu_gate_buf is a register buffer initialized before the loop.

The output has interleaved [silu(gate), silu(gate)*up] at granularity 8. The custom CUDA kernel extracts odd 8-col groups (the SwiGLU result) and quantizes to NVFP4 for the L2 GEMM.

The //2 Bug

interleave_l1_weights had g = granularity_bf16 // 2, correct for K-axis interleave (FP4 packing along K). But we interleave along N, where each N-column = 1 BF16 column. The //2 was a K-axis leftover that silently gave g=4 instead of g=8. Fixed: g = granularity_bf16 (no //2).

CuTeDSL Runtime Conditionals

CuTeDSL does support runtime conditionals on register tensors — both branches must produce the same tensor type (shape, layout, dtype). The earlier "blocked by type system" framing was wrong. The real issue: the old code applied SiLU to ALL positions (just SiLU, not SwiGLU) and the mask-blending approach (silu(both)*0.5) is mathematically wrong. With epi_tile_n=8 and subtile-level pairing, the conditional is clean.

The Global Scale Gotcha

The custom CUDA quantize kernel needs the L2 activation global scale (from the SwiGLU output), NOT the L1 input global scale. The L1 gs is based on the input magnitude (~0.1), while the SwiGLU output can be orders of magnitude larger. Passing the wrong gs causes the FP8 block scale to overflow, producing NaN. The runner pre-computes the L2 gs in compute_activation_global_scales() before CUDAGraph capture.

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Remaining Work

What	Status	Notes
In-epilogue NVFP4 quantize (replace BF16 TMA with FP4 TMA)	🔨 Future	Saves ~0.14ms/layer; requires register→GMEM mapping for FP4 output
GPU-native SWA decode attention	✅ Done	CuTeDSL kernel, cosine 0.9999+
GPU-native sparse + SWA decode attention	✅ Done	CuTeDSL kernel, cosine 0.9999+
vLLM Blackwell decode path	✅ Done	_attention_impl_blackwell uses native SWA + sparse kernels
Fuse fp8→bf16 dequant into CuTeDSL kernel	🔨 Future	Currently pre-dequantized on host; need vectorized fp8 loads
CSA/HCA sink weight merge in CuTeDSL	🔨 Future	Applied on host for now; fuse into kernel for perf

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DeepSeek-V4 Architecture Notes

NOT MLA. DeepSeek-V4 uses:

• CSA (Compressed Sparse Attention, cr=4): KV compressed 4x, indexer finds top-k
• HCA (Heavily Compressed Attention, cr=128): KV compressed 128x, pre-computed indices
• SWA: Standard sliding window (window=128, last layer only)
• mHC: Manifold-Constrained Hyper-Connections — replaces residual connections
• 384 experts, top-6, intermediate=3072

Compress ratios by layer: alternating 128/4, layer 60 = 0 (SWA).

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File Structure

cutedsl/
├── bridge.py                          # Quantization, layout, kernel launch
├── nvfp4_linear.py                    # Single-expert NVFP4 GEMM runner
├── runner.py                          # MoE grouped GEMM runner (fused + non-fused)
├── blackwell_attention.py             # KV cache + attention (standalone)
├── csa_attention.py                   # CSA/HCA attention
├── custom_ops.py                      # torch.autograd wrappers
├── moe_pipeline.py                    # Standalone test pipeline (fused + non-fused)
├── kernels/
│   └── deinterleave_quantize.cu      # Custom CUDA: de-interleave + NVFP4 quantize
└── kernel/moe/
    ├── torch_scaled_grouped_mm.py     # ScaledGroupedGemmKernel (the GEMM)
    └── fused_swiglu_grouped_mm.py     # FusedSwiGLUScaledGroupedGemmKernel

tests/
├── layertest.py                      # MoE layer test — fused + non-fused (PASS, 0.988)
├── cudagraph_test.py                  # CUDAGraph test (PASS)
├── test_full_layer_b200.py           # All NVFP4 projections (PASS, 0.994+)
├── test_v4_attention_b200.py         # All 3 attention types (PASS)
├── test_kv_cache_b200.py             # KV cache (PASS, 0.9997)
├── test_sparse_attn_b200.py          # CSA/HCA (PASS)
├── test_decode_attention_b200.py     # Prefill+decode (PASS, 0.9998)
└── ...

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Key Lessons

1. ⛔ NEVER assume CuTeDSL GPU tensors survive JIT compilation. cute.compile zeroes GPU memory. Keep index/mapping tensors on CPU.

2. ⛔ NEVER nuke working code without understanding why it exists. CUDAGraph-safe functions exist because vLLM requires CUDAGraph.

3. ⛔ NEVER fabricate facts from MEMORY.md. Verify what "works" means before citing it.

4. ⛔ NEVER quantize a padded buffer and slice the output. Quantize compact data, scatter into padded layout.

5. ⛔ Silent weight drops are deadly. vLLM's if name not in params_dict: continue skips weights with no warning. Replace with hard RuntimeError.

6. ⛔ NVFP4 is NOT suitable for attention Q×K^T. Per-element dot products are too sensitive. Keep attention in BF16.

7. ⛔ NEVER touch drivers, kernels, firmware, or system packages on the B200. The cluster costs millions. Always confirm with Mike.

8. ⛔ CuTeDSL if branches must produce the same tensor type. Both branches must yield identical (shape, layout, dtype). Initialize variables before the if — using values defined only inside a branch is not supported.

9. ⛔ The //2 in interleave was a K-axis leftover. FP4 packing is along K, not N. When interleaving along N, g = granularity_bf16 (no //2). The bug silently gave granularity 4 instead of 8.

10. ⛔ "SiLU on all positions" is NOT SwiGLU. SwiGLU pairs silu(gate)*up. Applying SiLU to the full (M, 2×intermediate) output is just SiLU. The pairing must be explicit.

11. ⛔ The global scale must match the data being quantized. Passing the L1 input gs to the SwiGLU quantize causes FP8 overflow → NaN. The gs must come from the SwiGLU output's magnitude.