Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN

[Paolo Bonzini]

On 1/17/22 10:55, Vitaly Kuznetsov wrote:
> No, honestly I was thinking about something much simpler: instead of
> forbidding KVM_SET_CPUID{,2} after KVM_RUN completely (what we have now
> in 5.16), we only forbid to change certain data which we know breaks
> some assumptions in MMU, from the comment:
> "
>           * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
>           * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
>           * tracked in kvm_mmu_page_role.  As a result, KVM may miss guest page
>           * faults due to reusing SPs/SPTEs.
> "
> It seems that CPU hotplug path doesn't need to change these so we don't
> need an opt-in/opt-out, we can just forbid changing certain things for
> the time being. Alternatively, we can silently ignore such changes but I
> don't quite like it because it would mask bugs in VMMs.

I think the version that only allows exactly the same CPUID is the best, 
as it leaves less room for future bugs.

Paolo