Re: [PATCH 1/3 V7] KVM, SEV: Add support for SEV intra host migration

[Sean Christopherson <seanjc@google.com>]

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.

> > > +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.