14.06.2021 20:06, Sean Christopherson пишет:
On Sun, Jun 13, 2021, stsp wrote:
Hi kvm developers.
I am having the strange problem that can only be reproduced on a core2duo CPU
but not AMD FX or Intel Core I7.
My code has 2 ways of setting the guest registers: one is the guest's ring0
stub that just pops all regs from stack and does iret to ring3. That works
fine. But sometimes I use KVM_SET_SREGS and resume the VM directly to ring3.
That randomly results in either a good run or invalid guest state return, or
a page fault in guest.
Hmm, a core2duo failure is more than likely due to lack of unrestricted guest.
You verify this by loading kvm_intel on the Core i7 with unrestricted_guest=0.
Wow, excellent shot!
Indeed, the problem then starts
reproducing also there!
So at least I now have a problematic
setup myself, rather than needing
to ask for ssh from everyone involved. :)
What does this mean to us, though?
That its completely unrelated to any
memory synchronization?
I tried to analyze when either of the above happens exactly, and I have a
very strong suspection that the problem is in a way I update LDT. LDT is
shared between guest and host with KVM_SET_USER_MEMORY_REGION, and I modify
it on host. So it seems like if I just allocated the new LDT entry, there is
a risk of invalid guest state, as if the guest's LDT still doesn't have it.
If I modified some LDT entry, there can be a page fault in guest, as if the
entry is still old.
IIUC, you are updating the LDT itself, e.g. an FS/GS descriptor in the LDT, as
opposed to updating the LDT descriptor in the GDT?
I am updating the LDT itself,
not modifying its descriptor
in gdt. And with the same
KVM_SET_SREGS call I also
update the segregs to the new
values, if needed.
Either way, do you also update all relevant segments via KVM_SET_SREGS after
modifying memory?
Yes, if this is needed.
Sometimes its not needed, and
when not - it seems page fault is
more likely. If I also update segregs -
then invalid guest state.
But these are just the statistical
guesses so far.
Best guess is that KVM doesn't detect that the VM has state
that needs to be emulated, or that KVM's internal register state and what's in
memory are not consistent.
Hope you know what parts are
emulated w/o unrestricted guest,
in which case we can advance. :)
Anyways, I highly doubt this is a memory synchronization issue, a corner case
related to lack of unrestricted guest is much more likely.
Just to be sure I tried the CD bit
in CR0 to rule out the caching
issues, and that changes nothing.
So...
What to do next?
