Messages in this thread | | | Subject | Re: [PATCH 07/54] KVM: x86: Alert userspace that KVM_SET_CPUID{,2} after KVM_RUN is broken | From | Paolo Bonzini <> | Date | Wed, 23 Jun 2021 21:53:14 +0200 |
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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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