Some GPU hangs witnessed in the wild on RDNA4 in Control and Arc Raiders
seem to point towards closest-hit shaders reading a stale value for the
SGPR pair containing the currently-executing shader's address.
This SGPR pair was read by VALU in the preceding traversal shader,
making it susceptible to VALUReadSGPRHazard. Inserting
VALUReadSGPRHazard mitigations before accessing the s_setpc target seems
to fix the hang. We don't have conclusive proof that this is hazardous,
but given that all signs point towards it and we have a reasonably
simple workaround, let's roll with this for now to mitigate the hangs.
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38290>
Natalie Vock
1243d575a5