linux-kernel - Re: [PATCH v2 2/2] KVM: x86: add MSR_KVM_MIGRATION

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Message-ID: <CABayD+dKLTx5kQTaKASQkcam4OiHJueuL1Vf32soiLq=torg+w@mail.gmail.com>
Date:   Wed, 28 Apr 2021 12:56:19 -0700
From:   Steve Rutherford <srutherford@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>, Joerg Roedel <joro@...tes.org>,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Venu Busireddy <venu.busireddy@...cle.com>
Subject: Re: [PATCH v2 2/2] KVM: x86: add MSR_KVM_MIGRATION_CONTROL

On Tue, Apr 27, 2021 at 3:14 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Wed, Apr 21, 2021, Paolo Bonzini wrote:
> > Add a new MSR that can be used to communicate whether the page
> > encryption status bitmap is up to date and therefore whether live
> > migration of an encrypted guest is possible.
> >
> > The MSR should be processed by userspace if it is going to live
> > migrate the guest; the default implementation does nothing.
> >
> > Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> > ---
>
> ...
>
> > @@ -91,6 +93,8 @@ struct kvm_clock_pairing {
> >  /* MSR_KVM_ASYNC_PF_INT */
> >  #define KVM_ASYNC_PF_VEC_MASK                        GENMASK(7, 0)
> >
> > +/* MSR_KVM_MIGRATION_CONTROL */
> > +#define KVM_PAGE_ENC_STATUS_UPTODATE         (1 << 0)
>
> Why explicitly tie this to encryption status?  AFAICT, doing so serves no real
> purpose and can only hurt us in the long run.  E.g. if a new use case for
> "disabling" migration comes along and it has nothing to do with encryption, then
> it has the choice of either using a different bit or bastardizing the existing
> control.
>
> I've no idea if such a use case is remotely likely to pop up, but allowing for
> such a possibility costs us nothing.
Using a different bit sounds fine to me. It would allow us to avoid
stuffing multiple meanings into a single bit, which would still happen
even if we had a better name.