Add ACCELERATOR_PORTING_SCOPE.md, the umbrella scope for taking the paged
backend's accelerator-portable wins off the CUDA family. It builds on (does
not duplicate) UPSTREAM_LAYER2_SCOPE.md, which stays the GDN/SSM-fusion
detail (benefit #1), and adds:
- Benefit #2 (paged KV in-kernel block-table flash-attn read, 0009-0011):
new per-backend feasibility from source analysis of the Metal/SYCL/Vulkan
flash-attn kernels. SYCL EASY (near line-for-line CUDA mirror), Metal
EASY-MEDIUM (decode already routes to the vec kernel), Vulkan MEDIUM (the
fast coopmat2 NVIDIA decode path cannot do the indexed read; push-constants
are full). Universal constraint: only the vec/scalar decode kernel admits
the per-cell indexed read, so route block-table ops onto vec (as CUDA's
0009-0010 dispatch guard already does) and leave the fast MM/coopmat2 path
contiguous-only. This is the lever that flips paged KV from
neutral-to-slightly-negative to non-negative off CUDA.
- Benefit #3 (decode-first scheduler, 0013/0016): confirmed a free portable
win - host-side update_slots() policy, zero kernel work, runs on any
accelerator as-is.
- Benefit #4 (NVFP4 FP4-MMA, 0017/0023/0025): out of scope (Blackwell only);
flags the backend-agnostic analogues of the act-quant dedup and the
graph-coverage lever without over-claiming a port.
- A ROCm note: ROCm rides the CUDA/HIP path (validate, don't re-port);
FP4-MMA stays Blackwell-only.
Benefits #1 and #2 share the port shape and rank Metal->SYCL->Vulkan, so they
bundle into one per-backend PR behind a shared ops-first PR. Cross-link added
from UPSTREAM_LAYER2_SCOPE.md. All gates are test-backend-ops on-target (no
Metal/SYCL/Vulkan/ROCm hardware here).
Assisted-by: Claude:opus-4.8 [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
