Hi Ralf,
I highly doubt BIOS HWI dispatcher can cause this stack corruption issue. If you look at the requirements for this issue to occur in Advisory 3.1.32, it requires 2 clean cache lines in the L1D. The HWI dispatcher first does a store of the CPU registers (thus dirtying the cache lines) before doing a parallel load of CPU registers when the HWI dispatcher returns. Since the cache lines are in a dirty state, the first Advisory 3.1.32 requirement is violated. Of course the dirty lines could get evicted but that seems highly unlikely.
Best,
Ashish