h3-blackwell-runtime/research/vortex_exact_attention/HARDWARE_CAPABILITIES.md

# Hardware Capabilities

## Interpretation Rule

Blackwell-native means using target-specific scheduling, state ownership, or
data movement supported by the target. Compiling the SM89 algorithm for
`sm_121a` is not sufficient. SM100, SM120, and SM121 are separate schedules.

## Capability Matrix

| Capability | SM100 / B200 | SM120 / RTX PRO 6000 | SM121 / GB10 |
| --- | --- | --- | --- |
| Compute capability | 10.0 | 12.0 | 12.1 |
| TMA global/shared tensor copies | Documented | Documented | Documented |
| Native matrix programming model | `tcgen05.mma` with TMEM | warp-level `mma.sync` | SM120-family warp-level `mma.sync` |
| Programmer-visible TMEM | Yes | Not exposed | Not exposed |
| Native UMMA/tcgen05 INT8 | Exposed | Not exposed | Not exposed |
| Sage-compatible INT8 path | `tcgen05.kind::i8` can be investigated | supported classic INT8 `mma.sync` | supported classic INT8 `mma.sync` |
| FP8 PV path | `tcgen05` FP8 forms | warp-level FP8 `mma.sync` | warp-level FP8 `mma.sync` |
| Hopper WGMMA | Not a portable target path | Not a target path | CUDA 13 rejects for `sm_121a` |
| Registers per SM / max per thread | 64K / 255 | 64K / 255 | 64K / 255 |
| Shared memory per SM / block | 228 / 227 KiB | 100 / 99 KiB | 100 / 99 KiB |
| Maximum resident warps | 64 | 48 | 48 |
| Maximum resident threads | 2048 | 1536 | 1536 |

The public SM120/121 Blackwell MMA interface does not expose a Sage2-compatible
UMMA INT8 QK operation. Exact SM121 work therefore retains the supported INT8
`mma.sync` arithmetic while making TMA staging, role scheduling, ownership, and
handoff Blackwell-specific. FP8/FP4 QK would define `vortex_fast`, not
`vortex_exact`.

## SM121 Facts From Retained Evidence

- The current 128-thread kernel uses 255 registers/thread and 32 KiB dynamic
  shared memory.
- Two CTAs provide eight resident warps, approximately `16.67%` of the 48-warp
  ceiling.
- A 168-register cap reaches three CTAs but spills catastrophically; compiler
  lifetime cleanup is not enough.
- CUDA 13 rejects `wgmma.fence`, `wgmma.mma_async`, `wgmma.commit_group`, and
  `wgmma.wait_group` for `sm_121a`.
- CUTLASS 4.6 executes SM120-family block-scaled narrow MMA on GB10, but that is
  not an exact replacement for Sage2 INT8 QK.
- Phase 2A measures VEA-B role resources, one-CTA residency, bounded mbarrier
  handoff, and concurrent INT8/FP8 issue. TMA integration remains a complete
  prototype concern rather than a measured capability claim.

## Capability Probe Status

1. Complete: 320-thread QK, PV, combined-ownership, handoff, and tensor-issue
   resource probes with zero local spills.
2. Complete: one 51.2 KiB ten-warp CTA/SM, confirmed by occupancy API and NCU.
3. Complete: corrected two-slot 591-epoch mbarrier handoff, deterministic and
   sanitizer-clean, with 48-block p50/p95 measurements.
4. Complete: simultaneous INT8 and FP8 warp progress in 48/48 blocks, plus an
   NCU tensor-issue capture.
5. Remaining for the aligned prototype: integrated TMA staging, bank-aware
   payload layout, complete-kernel resources, sustained clocks, and exact output.
6. Remaining before VEA-C: cluster/DSM support, placement, simultaneous
   residency, and remote shared-memory latency on GB10.

## Portability Policy

- SM121 is first and must compile specifically for `sm_121a`.
- SM120 may share API and contract code, but requires its own geometry,
  resource model, profiler evidence, and acceptance gates.
- SM100 should use a separate `tcgen05`/TMEM design. Porting the SM121 register
  accumulator schedule unchanged would discard the principal SM100 capability.

## Sources

- NVIDIA CUDA GPU Compute Capability: https://developer.nvidia.com/cuda-gpus
- CUDA Programming Guide: https://docs.nvidia.com/cuda/cuda-programming-guide/
- PTX ISA: https://docs.nvidia.com/cuda/parallel-thread-execution/
- CUTLASS Blackwell functionality:
  https://github.com/NVIDIA/cutlass/blob/main/media/docs/cpp/blackwell_functionality.md
- CUTLASS `mma_sm100_umma.hpp`, `mma_sm120.hpp`, and architecture config.
- Local measurements: `SAGE2_BLACKWELL_DESIGN.md` and retained P0/post-FC2 NCU
  reports.

Documented API support does not establish throughput, latency, queue depth, or
bit identity. Those remain target-specific measurements.