[Bug 215459] VM freezes starting with kernel 5.15

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=215459

Sean Christopherson (seanjc@google.com) changed:

           What    |Removed                     |Added
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                 CC|                            |seanjc@google.com

--- Comment #4 from Sean Christopherson (seanjc@google.com) ---
The fix Maxim is referring to is commit fdba608f15e2 ("KVM: VMX: Wake vCPU when
delivering posted IRQ even if vCPU == this vCPU").  But the buggy commit was
introduced back in v5.8, so it's unlikely that's the issue, or at least that
it's the only issue.  And assuming the VM in question has multiple vCPUs (which
I'm pretty sure is true based on the config), that bug is unlikely to cause the
entire VM to freeze; the expected symptom is that a vCPU isn't awakened when it
should be, and while it's possible multiple vCPUs could get unlucky, taking
down the entire VM is highly improbable.  That said, it's worth trying that
fix, I'm just not very optimistic :-)

Assuming this is something different, the biggest relevant changes in v5.15 are
that the TDP MMU is enabled by default, and that the APIC access page memslot
is not deleted when APICv is inhibited.

Can you try disabling the TDP MMU with APICv still enabled?  KVM allows that to
be toggled without unloading, e.g. "echo N | sudo tee
/sys/module/kvm/parameters/tdp_mmu", the VM just needs to be started after the
param is toggled.

Running v5.16 (or v5.16-rc8, as there are no KVM changes expected between rc8
ad the final release) would also be very helpful.  If we get lucky and the
issue is resolved in v5.16, then it would be nice to "reverse" bisect to
understand exactly what fixed the problem.

> Assuming I really do have APICv: is there anything I need to change in my XML
> to really make use of this feature or does it work "out of the box"?

APICv works out of the box, though lack of IOMMU support does mean that your
system can't post interrupts from devices, which is usually the biggest
performance benefit to APICv on Intel.

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