linux-kernel - Re: [PATCH v2 28/28] KVM: nVMX: Use cached host MSR_IA32_VMX

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Message-ID: <CALMp9eQ+L1HPXcw1mysx3tYeN-=mGrZCxjZWDcJwtdfZZ6z8Dg@mail.gmail.com>
Date:   Thu, 30 Jun 2022 05:27:52 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Anirudh Rayabharam <anrayabh@...ux.microsoft.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 28/28] KVM: nVMX: Use cached host MSR_IA32_VMX_MISC
 value for setting up nested MSR

On Thu, Jun 30, 2022 at 12:36 AM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Jim Mattson <jmattson@...gle.com> writes:
>
> > On Wed, Jun 29, 2022 at 8:07 AM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> >>
> >> vmcs_config has cased host MSR_IA32_VMX_MISC value, use it for setting
> >> up nested MSR_IA32_VMX_MISC in nested_vmx_setup_ctls_msrs() and avoid the
> >> redundant rdmsr().
> >>
> >> No (real) functional change intended.
> >
> > Just imaginary functional change? :-)
> >
>
> Well, yea) The assumption here is that MSR_IA32_VMX_MISC's value doesn't
> change underneath KVM, caching doesn't change anything then. It is, of
> course, possible that when KVM runs as a nested hypervisor on top of
> something else, it will observe different values. I truly hope this is
> purely imaginary :-)

It is also theoretically possible that a late-loadable microcode patch
could change the value of the MSR, but Intel wouldn't do that to us,
would they?