AstrAI/docs/developer/cuda_kernels.md

# CUDA Kernels

AstrAI includes optional custom CUDA kernels for attention, rotary embedding, and FP8 GEMM. These are built when `nvcc` is available and CUDA is detected, and are dispatched via the `CudaBackend` attention backend, auto-dispatched for rotary, or invoked through the FP8 linear primitives.

## Overview

| Kernel | File | Description |
|--------|------|-------------|
| `attn_decode` | `attention/decode.cu` | GQA decode attention (split-KV) |
| `attn_prefill` | `attention/prefill.cu` | GQA prefill attention (split-Q) |
| `attn_paged_decode` | `attention/paged_decode.cu` | Paged KV cache decode attention |
| `attn_paged_prefill` | `attention/paged_prefill.cu` | Paged KV cache prefill attention (ragged batch) |
| `rotary_emb` | `rotary/rotary_emb.cu` | Fused rotary embedding (cos/sin lookup + rotation) |
| `fp8_ops` | `fp8/ops.cu` | FP8 quantization + tensor-core GEMM (sm_89+) |

Additionally, optimized `.cuh` variants with tensor-core MMA (Matrix Multiply-Accumulate) exist:

| Variant | File | Optimization |
|---------|------|--------------|
| Split-KV MMA decode | `attention/decode_split_kv_mma.cuh` | Split KV across warps + MMA (sm_80+) |
| Split-Q MMA prefill | `attention/prefill_split_q_mma.cuh` | Split Q across warps + MMA (sm_80+) |

> The paged and non-paged paths share one kernel body. Prefill is templated on
> an independent Q schedule (`DenseQSchedule` / `PackedQSchedule`) and KV
> source (`ContigKV` / `PagedKV`); decode only needs the KV source. There are
> no separate `attn_paged_*.cuh` files.

### Rotary Embedding Kernel

The `rotary_emb` kernel (`csrc/kernels/rotary/rotary_emb.cu`) fuses cos/sin lookup and rotation into a single kernel:

- One thread per (head, dim-pair), vectorized `__nv_bfloat162` load/store
- f32 cos/sin input, bf16 compute and output
- 256-thread blocks, grid-stride loop
- Auto-dispatched via `apply_rotary_emb` in `astrai/extension/backend/rotary.py` (CUDA when available + inference mode, else torch complex-multiply fallback)
- No context-manager backend needed — rotary is backend-agnostic, both attention backends benefit

Standalone benchmark vs torch complex-multiply (48 calls = 24 layers × q+k): 6-9x faster, max diff 0 (decode) to 3e-2 (large prefill, bf16).

### FP8 GEMM / Linear Kernel

The `fp8_ops` family (`csrc/kernels/fp8/`) accelerates bf16 linear layers by
quantizing to FP8 and running tensor-core GEMMs (**requires sm_89+**; fp8
`mma.sync.m16n8k32` only exists on Ada/Hopper). It follows the same three-layer
style as attention, but split into **three** files:

| File | Role |
|------|------|
| `fp8/common.h` | `FP8Format` enum (E4M3/E5M2), `Fp8GemmTraits<Fmt, BlockM, BlockN, K, Stages>`, `Fp8GemmPolicy` (traits + layouts + scheduling knobs — the kernel's single template parameter), `FP8Params` POD — no torch |
| `fp8/quantize.cuh` | pure-CUDA device code: `fp8_quantize_kernel<Fmt, InT>` (bf16/fp16/fp32 → FP8 + amax, `quant_in_traits<InT>` vectorized unpack) — no torch |
| `fp8/gemm.cuh` | pure-CUDA device code: CUTLASS-style collectives (`Fp8GemmTileScheduler` / `Fp8CollectiveMainloop` / `Fp8CollectiveEpilogue`) around `fp8_gemm_kernel<Policy>` (pre-quantized GEMM; 64×64 / 128×64 / 128×128 CTA picked by `plan_gemm`, multi-stage cp.async, transposed-operand layouts, NN routed through a swap + out-transposed epilogue) — no torch. Entry: `gemm<Fmt>(params, stream, trans_a, trans_b)` = `canonicalize_gemm` → `plan_gemm` → `launch_plan` |
| `fp8/ops.cu` | binding only: `check_fp8_device` (sm_89+), param packing, launch dispatch, pybind → module `fp8_ops` |

Scale semantics: `quantize` takes the quantization *multiplier*; the
strategy layer passes `scale.reciprocal()` and the kernel multiplies by it.
`mm_fp8` takes the combined dequant scale (`sa * sb`). `amax` is always
returned in the original input domain.

Python layer (two levels): `astrai/extension/ops/fp8.py` provides stateless
primitives (`fp8_quantize` / `fp8_gemm`) via `torch.library.custom_op`, with
plain `quantize` / `mm_fp8` wrappers, and `astrai/extension/fp8.py` is the
strategy layer (`fp8_autocast`, delayed / dynamic scaling recipes,
`fp8_linear_forward/backward` wiring `aten::linear` on CUDA). See the FP8
section in `AGENTS.md` for full detail.

## Build System

### Auto-detection

Kernels are built when **both** of these conditions are met:
1. `nvcc` is available on `PATH`
2. `torch.cuda.is_available()` returns `True`

Unless `CSRC_KERNELS=false` is set explicitly.

### Manual build

```bash
# During install
CSRC_KERNELS=true pip install -e . --no-build-isolation

# Rebuild after editing .cu/.cuh files
CSRC_KERNELS=true python setup.py build_ext --inplace
# Output: astrai/extension/lib/*.so

# Or invoke CMake directly
cmake -S csrc -B build/cmake \
  -DTORCH_HOME=<site-packages>/torch \
  -DPYTHON_INCLUDE_DIR=<python include> \
  -DPY_SOABI=cpython-312-x86_64-linux-gnu
cmake --build build/cmake -j 16
```

### Architecture flags

`setup.py` passes the GPU compute capability to CMake via `ASTRAI_CUDA_ARCH`. When
unset, `setup.py` auto-detects the real GPU capability through
`torch.cuda.get_device_capability()`; the CMake fallback default is `80` (sm_80):

- **sm_80+** (Ampere and later): enables the tensor-core MMA path
  (`mma.sync.m16n8k16.bf16` for bf16 attention, `mma.sync.m16n8k32` for FP8).
- **sm_89+**: required for the FP8 family (`fp8_ops`) — FP8 tensor-core
  instructions only exist on Ada/Hopper and newer. On older architectures,
  CMake emits a warning and skips the `fp8_ops` target so the remaining CUDA
  kernels still build successfully.
- **`-DASTRAI_NO_MMA`** is a manual escape hatch only — the build never defines
  it automatically. To disable the MMA path, add it to `NVCC_FLAGS` yourself;
  all supported build targets are sm_80+.

### Build configuration

`csrc/CMakeLists.txt` defines the CUDA extension build:

```
NVCC_FLAGS = -O3 --expt-relaxed-constexpr --use_fast_math
             --ptxas-options=-O3,-v --extra-device-vectorization --threads=16
```

Each kernel in `astrai/extension/lib` is compiled as an independent pybind11 module (one `.so` per kernel, named `<kernel>.cpython-*-x86_64-linux-gnu.so`). CMake builds all registered kernel targets in parallel via `cmake --build -j N` (the five base targets always; `fp8_ops` additionally on sm_89+). The target list is the **single source of truth**: `KERNEL_NAMES` and the parallel `KERNEL_SRCS` list in `csrc/CMakeLists.txt`; `astrai/extension/loader.py` auto-discovers the compiled `.so` files.

## Python Extension Architecture

The Python extension package separates low-level kernel bindings from execution
policy:

```text
astrai/extension/
├── __init__.py             # Stable public API
├── loader.py               # Optional compiled-module discovery and loading
├── ops/
│   ├── attention.py        # Stateless attention kernel wrappers
│   ├── rotary.py           # Stateless rotary kernel wrapper
│   └── fp8.py              # Stateless FP8 primitives (custom_op)
├── fp8.py                  # FP8 strategy layer (fp8_autocast, recipes)
└── backend/
    ├── attention.py        # Backend selection, KV cache I/O, and fallback
    └── rotary.py           # Per-call CUDA/torch rotary dispatch
```

The dependency direction is one-way:

```text
model / inference
       |
       v
extension public API
       |
       v
backend policy  --->  ops wrappers  --->  loader  --->  compiled .so
       |
       +----------->  torch / flash-attn fallback
```

`ops` must not import `backend`. This keeps direct kernel bindings independent
of model, cache, fallback, and backend-selection policy.

### Ops Layer

`astrai.extension.ops` is the low-level boundary around compiled extensions:

- Wrappers are stateless and map Python arguments to pybind or
  `torch.library.custom_op` calls.
- Wrappers validate kernel availability and raise `RuntimeError` when a
  requested extension was not built.
- Wrappers do not choose another implementation, gather KV cache entries, or
  decide whether an input is supported by a backend.
- Tests that specifically exercise a compiled kernel may import from
  `astrai.extension.ops`.

For example, `attn_prefill(...)` means "run this CUDA kernel" rather than "run
attention using the best available implementation":

```python
from astrai.extension.ops import attn_prefill

output = attn_prefill(q, k, v, mask=mask, is_causal=True)
```

If the kernel is unavailable, this call fails. Callers that need fallback and
capability dispatch must use the public `attention(...)` entry point instead.

### Backend Layer

`astrai.extension.backend` owns execution policy:

- It selects CUDA, FlashAttention, or torch-native attention.
- It checks per-call constraints such as dtype, shape, head dimension, cache
  availability, and installed optional dependencies.
- It owns KV cache writes and reads because those operations differ by backend.
- It provides torch fallbacks and raises when an explicitly requested backend
  cannot handle a call.
- Rotary dispatch follows the same boundary without a backend class: the
  policy layer chooses the fused op for supported inference calls and otherwise
  uses the autograd-compatible torch implementation.

Normal model and inference code should import the stable API from
`astrai.extension`:

```python
from astrai.extension import ATTN_BACKEND, attention, attn_backend

output = attention(q, k, v, kv_cache=cache, layer_id=layer_id, fwd="decode")

with attn_backend(ATTN_BACKEND.TORCH_NATIVE):
    output = attention(q, k, v)
```

The package root re-exports the supported high-level API and selected direct
kernel wrappers. Internal code should use `astrai.extension.backend` only when
it needs a backend type or policy implementation, and `astrai.extension.ops`
only when it deliberately requires one exact kernel.

### Placement Rules

When extending this package:

| Change | Location |
|--------|----------|
| Add a pybind call for a compiled kernel | `astrai/extension/ops/` |
| Add argument translation required by the compiled ABI | `astrai/extension/ops/` |
| Add capability checks or implementation selection | `astrai/extension/backend/` |
| Add a torch or third-party fallback | `astrai/extension/backend/` |
| Add attention KV cache behavior | `astrai/extension/backend/attention.py` |
| Expose a supported user-facing symbol | `astrai/extension/__init__.py` |

Imports belong at module scope. Optional dependencies such as `flash_attn` may
use a module-level guarded import. Type-only imports that would create a runtime
cycle belong under `TYPE_CHECKING`.

## Attention Backend

`astrai/extension/backend/attention.py` provides the backend abstraction:

- **`AttentionBackend`** (ABC): `fwd_decode` / `fwd_prefill` abstract methods, `forward` dispatches by q_len
- **`CudaBackend`**: CUDA kernel dispatch — decode via `attn_paged_decode` (page_size=1), prefill via `attn_paged_prefill` (ragged batch, `qo_indptr` + `kv_indptr`). Default on GPU.
- **`FlashAttnBackend`**: Optional flash-attn dispatch with `flash_attn_with_kvcache` fast path.
- **`TorchNativeBackend`**: SDPA with indirect KV cache gather (always-available fallback)

Default priority: cuda > flash > torch. Set ``ASTR_BACKEND=cuda|torch_native|flash``
to override the default.

Select a backend via context manager (mirrors `torch.nn.attention.sdpa_kernel`):

```python
from astrai.extension import attn_backend, ATTN_BACKEND

with attn_backend(ATTN_BACKEND.CUDA):
    engine.generate("hello")
```

The `attention(...)` policy entry point falls back to `FlashAttnBackend` (when
flash-attn is installed and supports the call) or `TorchNativeBackend` when the
automatically selected CUDA backend cannot handle an input. Resolution
precedence is: explicit `attn_backend(...)` context > `ASTR_BACKEND` env >
default. An explicit `attn_backend(...)` selection is strict and raises instead
of silently switching implementations; the env override (and the implicit
default) fall back to the first compatible backend when incapable. Training
calls (`fwd=None`, no KV cache) resolve by capability: the CUDA cache kernels
cannot run without a cache, so they fall back to flash (mask-free/causal calls
only) and finally to torch SDPA.

### Rotary Backend

`astrai/extension/backend/rotary.py` provides `apply_rotary_emb(x, (cos, sin))` with auto-dispatch:

- **CUDA path**: calls `rotary_emb` kernel directly when available, input is bf16 on CUDA, and `torch.is_grad_enabled()` is `False` (inference)
- **Torch fallback**: complex multiply (`torch.view_as_complex` → `torch.complex` multiply → `torch.view_as_real`), used during training (supports autograd) or when kernel unavailable

No context-manager switching needed — the dispatch is automatic per call.

## Python Wrappers

`astrai/extension/ops/attention.py` provides Python wrappers for each compiled attention kernel. Each wrapper calls its CUDA kernel directly and raises `RuntimeError` if the `.so` is not available. Fallback to torch SDPA is handled by the attention backend, not the wrapper functions.

`astrai/extension/ops/rotary.py` provides the wrapper for the rotary embedding kernel. Fallback to torch complex multiply is handled by `backend/rotary.py`.

Interface (all functions):
```
is_causal: True = causal mask; False = non-causal
mask:      2D [batch, kv_len] or 3D [batch, q_len, kv_len] (bool, True=keep)
```

Layout convention: all q/k/v are `[batch, seq_len, n_heads, head_dim]` (blhd). Scale is always `1/sqrt(head_dim)`.

### Q Scheduling and KV Addressing

Prefill separates Q work scheduling from KV storage:

- `DenseQSchedule` maps a rectangular grid directly with
  `batch = blockIdx.z` and `q_tile = blockIdx.x`.
- `PackedQSchedule` consumes a compact work map for a packed
  `[total_q, q_heads, head_dim]` tensor.
- `ContigKV` and `PagedKV` only provide KV lengths and translate logical KV
  positions into physical addresses. They do not schedule Q blocks.

For ragged Q lengths `[70, 10, 130]` and 64 rows per Q tile, cache binding
builds:

```text
qo_indptr       = [0, 70, 80, 210]
q_tile_to_batch = [0, 0, 1, 2, 2, 2]
q_tile_to_index = [0, 1, 0, 0, 1, 2]
```

Paged prefill launches:

```text
grid.x = num_q_tiles  # 6, exactly the valid ragged work items
grid.y = q_heads
grid.z = 1
```

Each block resolves its request and request-local tile in O(1):

```cpp
batch = q_tile_to_batch[blockIdx.x];
q_tile = q_tile_to_index[blockIdx.x];
```

The kernel then uses `qo_indptr[batch]` for the packed Q base and adjacent
`qo_indptr` / `kv_indptr` entries for that request's Q and KV lengths. This
avoids the previous per-block linear scan over the batch, shared-memory
broadcast, mapping barrier, and upper-bound grid with potentially invalid
blocks.

## Standalone Testing

Each `csrc/tests/*.cu` file has the `nvcc` compile command in its header comment. Example:

```bash
nvcc -I csrc -arch=sm_89 -O3 --use_fast_math \
     --ptxas-options=-O3,-v --extra-device-vectorization \
     -Xcompiler -fopenmp csrc/tests/attn_test.cu -o /tmp/test && /tmp/test
```

Test files:
- `attn_test.cu` — decode + prefill kernels (correctness tables + benchmarks)
- `attn_paged_test.cu` — paged decode/prefill kernels
- `fp8_mma_test.cu` — BF16→FP8→BF16 MMA demo (sm_89)

## Benchmarks

Hardware: NVIDIA L20 (sm_89, 46 GB), CUDA 12.8, driver 570.86.

Reproduce (decode + prefill in `attn_test.cu`, paged in `attn_paged_test.cu`):
```bash
nvcc -I csrc -arch=sm_89 -O3 --use_fast_math \
     --ptxas-options=-O3,-v --extra-device-vectorization \
     -Xcompiler -fopenmp csrc/tests/attn_test.cu -o /tmp/test && /tmp/test
```

## Known Optimization Targets

- **Decode D=256**: spill eliminated (BC=16 + STAGES=2), but still 248 regs — further tiling could help.
- **Prefill single-batch**: bandwidth low (22 GB/s at q=kv=2048) — compute-bound at ~94 TFLOP/s (near L20 bf16 ceiling ~193 TFLOP/s for non-causal).
- **Decode single-batch**: bandwidth low (113 GB/s at kv=512, 13% of 864 GB/s theoretical) — small kv underutilizes SMs despite split-KV; scales to 757 GB/s (88%) at B=16+.

## File Layout

```
csrc/
├── CMakeLists.txt                    # CMake build: kernel registry (KERNEL_NAMES / KERNEL_SRCS), torch/pybind11 linking
├── kernels/
│   ├── common/                       # cross-family pure-CUDA helpers (no torch)
│   │   ├── device.cuh                #   sm_at_least(), kMinSmForFp8* constants
│   │   └── mma.cuh                   #   shared mma_sync<InT> + mma_shape<InT> (bf16 m16n8k16 / fp8 m16n8k32) + ldmatrix_x2/x4<T>
│   ├── attention/                    # attention family (module names keep the attn_* prefix)
│   │   ├── common.h                  #   AttentionParams POD, TensorLayout enum (BHLD/BLHD)
│   │   ├── warp_utils.cuh            #   warp reduction helpers
│   │   ├── layout_policies.cuh       #   KV addressing policies: DenseQSchedule/PackedQSchedule, ContigKV/PagedKV
│   │   ├── mma_utils.cuh             #   ldmatrix/pack helpers + online-softmax (bf16 mma via common/mma.cuh)
│   │   ├── entry_utils.cuh           #   torch binding helpers: DISPATCH_HEAD_DIM, pack_*_params
│   │   ├── dispatchers.cuh           #   pure-CUDA launchers: dispatch_decode/prefill (+paged), split-K math
│   │   ├── decode_split_kv.cuh       #   decode kernel, scalar (split-KV)
│   │   ├── decode_split_kv_mma.cuh   #   decode kernel, MMA + split-K
│   │   ├── prefill_split_q.cuh       #   prefill kernel, scalar (split-Q)
│   │   ├── prefill_split_q_mma.cuh   #   prefill kernel, MMA (split-Q, packed/ragged Q schedule)
│   │   ├── decode.cu                 #   → module attn_decode
│   │   ├── prefill.cu                #   → module attn_prefill
│   │   ├── paged_decode.cu           #   → module attn_paged_decode
│   │   └── paged_prefill.cu          #   → module attn_paged_prefill
│   ├── rotary/
│   │   └── rotary_emb.cu             # rotary embedding (kernel + binding in one file) → module rotary_emb
│   └── fp8/                          # FP8 family (module name fp8_ops)
│       ├── common.h                  #   FP8Format enum, Fp8GemmTraits, FP8Params POD (no torch)
│       ├── gemm.cuh                  #   FP8 device code: quantize + pre-quantized GEMM kernels (no torch)
│       └── mm.cu                     #   binding only: validation, param packing, launch dispatch, pybind
└── tests/
    ├── test_utils.cuh                # Shared test utilities (now_ms, f2bf, bf2f, randf)
    ├── attn_test.cu                  # Decode + prefill kernels
    ├── attn_paged_test.cu            # Paged decode/prefill kernels
    └── fp8_mma_test.cu               # BF16→FP8→BF16 MMA demo
```

Compiled `.so` files are placed in `astrai/extension/lib/`, separate from Python source files.

> Document Update Time: 2026-08-22