[BUG] Guest OSes die simultaneously (bisected)

[Paul E. McKenney]

Hello!

Since some time between v5.19 and v6.4, long-running rcutorture tests
would (rarely but intolerably often) have all guests on a given host die
simultaneously with something like an instruction fault or a segmentation
violation.

Each bisection step required 20 hosts running 10 hours each, and
this eventually fingered commit c59a1f106f5c ("KVM: x86/pmu: Add
IA32_PEBS_ENABLE MSR emulation for extended PEBS").  Although this commit
is certainly messing with things that could possibly cause all manner
of mischief, I don't immediately see a smoking gun.  Except that the
commit prior to this one is rock solid.

Just to make things a bit more exciting, bisection in mainline proved
to be problematic due to bugs of various kinds that hid this one.  I was
therefore forced to bisect among the commits backported to the internal
v5.19-based kernel, which fingered the backported version of the patch
called out above.

Please note that this is not (yet) an emergency.  I will just continue
to run rcutorture on v5.19-based hypervisors in the meantime.

Any suggestions for debugging or fixing?

							Thanx, Paul