linux-kernel - Re: [PATCH] KVM: SEV: Init target VMCBs in sev_migrate

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Date:   Wed, 22 Jun 2022 13:33:32 -0600
From:   Peter Gonda <pgonda@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm list <kvm@...r.kernel.org>, Marc Orr <marcorr@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: SEV: Init target VMCBs in sev_migrate_from

> >
> > +void sev_init_vmcb(struct vcpu_svm *svm)
> > +{
> > +     svm->vmcb->control.nested_ctl |= SVM_NESTED_CTL_SEV_ENABLE;
> > +     clr_exception_intercept(svm, UD_VECTOR);
>
> I don't love separating SEV and SEV-ES VMCB initialization, especially since they're
> both doing RMW operations and not straight writes.  E.g. migration ends up reversing
> the order between the two relatively to init_vmcb().  That's just asking for a subtle
> bug to be introduced that affects only due to the ordering difference.
>
> What about using common top-level flows for SEV and SEV-ES so that the sequencing
> between SEV and SEV-ES is more rigid?  The resulting sev_migrate_from() is a little
> gross, but IMO it's worth having a fixed sequence, and the flip side to the ugliness
> it that it documents some of the differences between SEV and SEV-ES migration.

Thanks for the suggestion Sean! I like your suggestion here. I'll test
it out, clean it up and send it out as V2. I think the distinction
between SEV and SEV-ES migration was largely due to how I split up the
set of patches that enabled this feature.