Re: [PATCH v2 28/28] KVM: nVMX: Use cached host MSR_IA32_VMX_MISC value for setting up nested MSR

[Jim Mattson]

On Thu, Jun 30, 2022 at 12:36 AM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>
> Jim Mattson <jmattson@xxxxxxxxxx> writes:
>
> > On Wed, Jun 29, 2022 at 8:07 AM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> 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?