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## Motivation
The existing paged-KV attention pipelines (pagedkv, splitkv) support
StreamLLM-style sink tokens — a fixed set of initial tokens kept in
attention alongside the sliding window. The `batch_prefill` pipeline
(chunked-prefill with VLLM-style block tables) previously hardcoded
`kHasSink = false`, making it incompatible with sink-based attention
patterns in LLM serving scenarios.
This PR extends `batch_prefill` to support `kHasSink` and wires it
into `fmha_fwd_runner` for validation against the existing CPU
reference.
## Technical Details
**Pipeline** (`block_fmha_batch_prefill_pipeline_qr_ks_vs_async.hpp`):
- When `kHasSink`, the K/V loop splits into a sink phase [0,
sink_seq_end)
and a window phase [seqlen_k_start, seqlen_k_end), mirroring pagedkv.
- K advance at the sink→window transition jumps
`seqlen_k_start - sink_seq_end + kN0` to bridge the gap.
- V scatter-gather offsets are re-initialized at the transition to fix a
window mismatch bug: V was lagging kN0 behind K after the large jump,
loading from the wrong sequence position.
- Bias window, dropout seq_offset, and mask type (LogitsSinkMask)
updated
for sink-awareness.
**Traits / codegen** (`tile_fmha_traits.hpp`, `fmha_fwd.hpp`,
`fmha_batch_prefill.py`):
- `TileFmhaBatchPrefillTraits` gains `kHasSink_` (was hardcoded
`false`).
- Codegen adds `F_sink` field; skips batch-mode kernels (group mode
required).
- CMake test filter broadened from 9 → 33 instances covering
fp16/bf16 × mask/nmask × lse/nlse × sink/nsink.
**Runner** (`fmha_fwd_runner.hpp`, `CMakeLists.txt`):
- `fmha_batch_prefill()` dispatched from `run_fwd` when:
group mode + paged KV + num_splits == 1.
- K/V strides corrected for runner's [num_pages, nhead_k,
page_block_size, hdim] layout.
- `page_block_size % 128` check relaxed: batch_prefill supports ps=16.
- CPU reference paged-KV reordering guards extended with
`CK_TILE_FMHA_FWD_BATCH_PREFILL_API`.
## Test Plan
Build with `-DFMHA_FWD_ENABLE_APIS="fwd;batch_prefill"`, run
`tile_example_fmha_fwd` in group mode with page_block_size=16.
Test matrix:
- Mask: no-mask, causal, sliding window
- Sink: nsink, sink=1..128
- dtype: fp16, bf16
- LSE output: on/off
- seqlen ∈ {512,1024,2048,4096} × window ∈ {32,256,512,1024}
- GQA, chunked prefill, large batch×seqlen
- page_block_size: 16, 32
## Test Result
171 test cases, all valid:y:
- nmask + nsink: ✓
- causal + nsink: ✓
- causal + sink=8: ✓
- sliding window + sink=8 (d=128, d=256): ✓
- bf16, LSE output, GQA: ✓
## Submission Checklist
- [ ] Look over the contributing guidelines at
https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.