exception vs SIGALRM race (was: Re: guest/host mem out of sync on core2duo?)

[stsp <stsp2@yandex.ru>]

19.06.2021 00:07, Jim Mattson пишет:
> I believe DS is illegal. Per the SDM, Checks on Guest Segment Registers:
OK, so this indeed have solved
the biggest part of the problem,
thanks again.

Now back to the original problem,
where I was getting a page fault
on some CPUs sometimes.
I digged a bit more.
It seems I am getting a race of
this kind: exception in guest happens
at the same time when the host's
SIGALRM arrives. KVM returns to
host with the exception somehow
"pending", but its still on ring3, not
switched to the ring0 handler.

Then from host I inject the interrupt
(which is what SIGALRM asks for),
and when I enter the guest, it throws
the pending exception instead of
executing the interrupt handler.
I suspect the bug is again on my side,
but I am not sure how to handle that
kind of race. I suppose I need to look
at some interruptibility state to find
out that the interrupt cannot be injected
at that time. But I can't find if KVM
exports the interruptibility state, other
than guest's IF/VIF flag, which is not
enough in this case.
Also I am a bit puzzled why I can't
see such race on an I7 CPU even
after disabling the unrestricted_guest.

Any ideas? :)