The 8-mode indexing (tBgK[None,None,None,None,kt,None,None,None]) fails at
JIT compilation with 'coord and shape are weakly congruent' error. The actual
MLIR tensor shape is (((64,128),1),?,?,?) — 4 modes, not 8.
The working fix from commit 845ad98 on the B200 used 4-mode indexing all along:
tBgK[(None, None, kt, 0)] — mode 2 = GMEM tile dim
tVgV[(None, 0, kt, 0)] — mode 2 = GMEM tile dim
Updated all files: example10, test_fmha_v3_stage_c, README, docstrings.
K from QK MMA B-partition has GMEM iter at mode 1, NOT mode 2.
(None,0,None,0) hardcodes mode 1 to 0 → TMA always loads tile 0.
(None,None,0,0) keeps mode 1 free → correct multi-tile loading.
Proof: diag n=256 went from cos 0.711 → 0.999999 with this one change.
cute.size() returns a CuTeDSL symbol, not a Python int.
range() on a symbol can't iterate — the loop never unrolls.
Now n_kv_tiles is computed in __init__ as s_k // 128 (Python int).
cutlass.range traces once - kv_coord/kt are trace-time values,
not runtime loop-carried state. Python range() fully unrolls at
trace time, emitting distinct Int32(k) constants per iteration.
Int32(1) hardcoded already proved TMA CAN load from tile 1.
Key findings to relay to CUTLASS LLM:
- kv_coord=Int32(1) hardcode CHANGES the output (TMA CAN load from different tiles)
- kv_coord=Int32(0) + kv_coord += 1 does NOT increment at runtime
(all multi-tile outputs identical to kv_coord=0)
- kv_coord=0 (plain Python int) also doesn't work
- Pipeline handle .count doesn't work either
- The TMA GMEM tile coordinate must be dynamic at kernel runtime,
but CuTeDSL appears to constant-fold or not propagate the increment
Both GMEM and SMEM sides must be sliced to the same rank for cute.copy.
K (QK MMA B-partition): slice [(None,None,0,0)] keeps modes 0,1
- mode 1 = GMEM iteration, indexed by kvh.count
V (PV MMA B-partition): slice [(None,0,None,0)] keeps modes 0,2
- mode 2 = GMEM iteration, indexed by kvh.count
Q: only 1 tile, (None,0,None,0) hardcode is fine.
Root cause of multi-tile failure: (None,0,None,0) slice hardcodes the
GMEM tile dimension to 0, so TMA always loads from tile 0 regardless
of kvh.count. K from QK MMA has GMEM iter at mode 1, V from PV MMA
has it at mode 2 (different layouts: K,D,L vs D,K,L).
Fix follows CUTLASS FMHA reference:
- K: tBgK[(None,None,None,0)] + tBgK[(None, kvh.count, None)]
- V: tVgV[(None,0,None,0)] + tVgV[(None, kvh.count)]
row_max is already in log2(scaled) space (S * scale_log2), so
old_row_max - row_max_safe is the correct exponent for exp2.
The old code computed exp2(scale_log2 * (old_row_max - row_max_safe))
which is exp2(scale_log2^2 * (old_max_S - new_max_S)) — wrong.
- Combined K+V barrier (one acquire per kt, kvh.count == kt)
- O rescale for kt > 0 (online softmax O correction)
- final_o_bar sync (MMA signals before producer_tail)
- s_k as constructor param (compile-time for V layout)
- kv_tx_bytes covers both K and V transfers
- Test covers n=128, 256, 512, 1024
row_max is in scaled domain (s_val * scale_log2). The O rescaling
should be exp2(old_max - new_max) without extra scale_log2 because
the max values already include the scaling factor.
row_max should be the max of the raw QK scores, not pre-scaled.
The scale_log2 is applied during exp2 and rescaling, not stored in row_max.
This fixes the double-scaling bug that broke multi-tile O rescaling.
