Re: [PATCH 07/54] KVM: x86: Alert userspace that KVM_SET_CPUID{,2} after KVM_RUN is broken

[Paolo Bonzini]

On 23/06/21 21:02, Jim Mattson wrote:
> > BTW, there is actually a theoretical usecase for KVM_SET_CPUID2 after
> > KVM_RUN, which is to test OSes against microcode updates that hide,
> > totally random example, the RTM bit.  But it's still not worth keeping
> > it given 1) the bugs and complications in KVM, 2) if you really wanted
> > that kind of testing so hard, the fact that you can just create a new
> > vcpu file descriptor from scratch, possibly in cooperation with
> > userspace MSR filtering 3) AFAIK no one has done that anyway in 15 years.
>
> Though such a usecase may exist, I don't think it actually works
> today. For example, kvm_vcpu_after_set_cpuid() potentially changes the
> value of the guest IA32_PERF_GLOBAL_CTRL MSR.

Yep, and that's why I'm okay with actively deprecating KVM_SET_CPUID2 
and not just "discouraging" it.

Paolo