Re: [PATCH v12 12/29] KVM: SEV: Add KVM_SEV_SNP_LAUNCH_FINISH command

[Paolo Bonzini]

On Tue, Apr 2, 2024 at 1:18 AM Michael Roth <michael.roth@xxxxxxx> wrote:
>
> On Sat, Mar 30, 2024 at 09:41:30PM +0100, Paolo Bonzini wrote:
> > On 3/29/24 23:58, Michael Roth wrote:
> > >
> > > +           /* Handle boot vCPU first to ensure consistent measurement of initial state. */
> > > +           if (!boot_vcpu_handled && vcpu->vcpu_id != 0)
> > > +                   continue;
> > > +
> > > +           if (boot_vcpu_handled && vcpu->vcpu_id == 0)
> > > +                   continue;
> >
> > Why was this not necessary for KVM_SEV_LAUNCH_UPDATE_VMSA?  Do we need it
> > now?
>
> I tried to find the original discussion for more context, but can't seem to
> locate it. But AIUI, there are cases where a VMM may create AP vCPUs earlier
> than it does the BSP, in which case kvm_for_each_vcpu() might return an AP
> as it's first entry and cause that VMSA to get measured before, leading
> to a different measurement depending on the creation ordering.

I think that would be considered a bug in either the VMM or the
"thing" that computes the measurement.

If that hasn't been a problem for SEV-ES, I'd rather keep the code simple.

> We could however limit the change to KVM_X86_SEV_ES_VM and
> document that as part of KVM_SEV_INIT2, since there is similarly chance
> for measurement changes their WRT to the new FPU/XSAVE sync'ing that was
> added.

Hmm, I need to double check that the FPU/XSAVE syncing doesn't break
existing measurements, too.

Paolo