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

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