Does anybody have any other ideas?

the only other weird case that comes to mind; what happens if there's a line 
dirty in the caches,
but the memory is now mapped uncached. (Which could happen if kexec does muck 
with  MTRRs, CR0 or other similar
things in weird ways)... not sure what happens in CPU, a machine check for 
cache inclusion violations
is not beyond the imagination and might be lethal

this would explain a kexec specific angle versus general normal (but rare) use 
of wbinvd.


other weird case could be cached mmio (not common, but some gpus and the like 
can do it)
with iommu/VT-D in the middle, and during kexec VT-D shutting
down the iommu before the wbinvd. This would be... highly odd... but this 
report already is in highly odd space.

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