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From:   Peter Gonda <pgonda@google.com>
Date:   Fri, 10 Sep 2021 16:07:44 -0600
Message-ID: <CAMkAt6qQkPcTz=PZfVYtx2EmoLFvpDaq6eNDx9cx6o3ZGyLCaw@mail.gmail.com>
Subject: Re: [PATCH 1/3 V7] KVM, SEV: Add support for SEV intra host migration
To:     Sean Christopherson <seanjc@google.com>
Cc:     kvm list <kvm@vger.kernel.org>, Marc Orr <marcorr@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        David Rientjes <rientjes@google.com>,
        "Dr . David Alan Gilbert" <dgilbert@redhat.com>,
        Brijesh Singh <brijesh.singh@amd.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org
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On Fri, Sep 10, 2021 at 4:03 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Fri, Sep 10, 2021, Peter Gonda wrote:
> > > Do we really want to bury this under KVM_CAP?  Even KVM_CAP_VM_COPY_ENC_CONTEXT_FROM
> > > is a bit of a stretch, but at least that's a one-way "enabling", whereas this
> > > migration routine should be able to handle multiple migrations, e.g. migrate A->B
> > > and B->A.  Peeking at your selftest, it should be fairly easy to add in this edge
> > > case.
> > >
> > > This is probably a Paolo question, I've no idea if there's a desire to expand
> > > KVM_CAP versus adding a new ioctl().
> >
> > Thanks for the review Sean. I put this under KVM_CAP as you suggested
> > following the idea of svm_vm_copy_asid_from. Paolo or anyone else have
> > thoughts here? It doesn't really matter to me.
>
> Ah, sorry :-/  I obviously don't have a strong preference either.

I am going to suggest leaving it under KVM_CAP for this reason. I
don't see a great use case for A->B then B->A migrations. And if we
are going to move to dst must be not SEV or SEV-ES enabled, which I
think makes sense. Then your VM can only ever have migrated from 1
other VM since once it has it will be SEV/SEV-ES enabled. Does that
seem reasonable?

>
> > > > +Architectures: x86 SEV enabled
> > > > +Type: vm
> > > > +Parameters: args[0] is the fd of the source vm
> > > > +Returns: 0 on success
> > >
> > > It'd be helpful to provide a brief description of the error cases.  Looks like
> > > -EINVAL is the only possible error?
> > >
> > > > +This capability enables userspace to migrate the encryption context
> > >
> > > I would prefer to scope this beyond "encryption context".  Even for SEV, it
> > > copies more than just the "context", which was an abstraction of SEV's ASID,
> > > e.g. this also hands off the set of encrypted memory regions.  Looking toward
> > > the future, if TDX wants to support this it's going to need to hand over a ton
> > > of stuff, e.g. S-EPT tables.
> > >
> > > Not sure on a name, maybe MIGRATE_PROTECTED_VM_FROM?
> >
> > Protected VM sounds reasonable. I was using 'context' here to mean all
> > metadata related to a CoCo VM as with the
> > KVM_CAP_VM_COPY_ENC_CONTEXT_FROM. Is it worth diverging naming here?
>
> Yes, as they are two similar but slightly different things, IMO we want to diverge
> so that it's obvious they operate on different data.

Sounds good I'll rename.