linux-kernel - Re: [PATCH 07/54] KVM: x86: Alert userspace that KVM_SET_CPUID{,2} after KVM

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Date:   Wed, 23 Jun 2021 21:53:14 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH 07/54] KVM: x86: Alert userspace that KVM_SET_CPUID{,2}
 after KVM_RUN is broken

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