linux-kernel - Re: [PATCH] KVM: SEV: Disable KVM_CAP_VM_COPY_ENC_CONTEXT

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Date:   Tue, 14 Sep 2021 18:49:35 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Gonda <pgonda@...gle.com>
Cc:     kvm list <kvm@...r.kernel.org>, Marc Orr <marcorr@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Nathan Tempelman <natet@...gle.com>,
        Brijesh Singh <brijesh.singh@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: SEV: Disable KVM_CAP_VM_COPY_ENC_CONTEXT_FROM for
 SEV-ES

On Tue, Sep 14, 2021, Peter Gonda wrote:
> On Tue, Sep 14, 2021 at 12:41 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > -stable, for giggles
> >
> > On Tue, Sep 14, 2021, Peter Gonda wrote:
> > > On Tue, Sep 14, 2021 at 11:32 AM Sean Christopherson <seanjc@...gle.com> wrote:
> > > >
> > > > On Tue, Sep 14, 2021, Peter Gonda wrote:
> > > > > Copying an ASID into new vCPUs will not work for SEV-ES since the vCPUs
> > > > > VMSAs need to be setup and measured before SEV_LAUNCH_FINISH. Return an
> > > > > error if a users tries to KVM_CAP_VM_COPY_ENC_CONTEXT_FROM from an
> > > > > SEV-ES guest.
> > > >
> > > > What happens if userspace does KVM_CAP_VM_COPY_ENC_CONTEXT_FROM before the source
> > > > has created vCPUs, i.e. before it has done SEV_LAUNCH_FINISH?
> > >
> > > That's not enough. If you wanted to be able to mirror SEV-ES you'd
> > > also need to call LAUNCH_UPDATE_VMSA on the mirror's vCPUs before
> > > SEV_LAUNCH_FINISH. That is do-able but I was writing a small change to
> > > fix this bug. If mirroring of SEV-ES is wanted it's a much bigger
> > > change.
> >
> > Is it doable without KVM updates?  If so, then outright rejection may not be the
> > correct behavior.
> 
> I do not think so. You cannot call KVM_SEV_LAUNCH_UPDATE_VMSA on the mirror
> because svm_mem_enc_op() blocks calls from the mirror. So either you have to
> update vmsa from the mirror or have the original VM read through its mirror's
> vCPUs when calling KVM_SEV_LAUNCH_UPDATE_VMSA. Not sure which way is better
> but I don't see a way to do this without updating KVM.

Ah, right, I forgot all of the SEV ioctls are blocked on the mirror.  Put something
to that effect into the changelog to squash any argument about whether or not this
is the correct KVM behavior.

Thanks!