Re: [RFC PATCH v2 12/12] i386/sev: update query-sev QAPI format to handle SEV-SNP

[Dr. David Alan Gilbert]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Wed, Sep 01, 2021 at 04:14:10PM +0200, Markus Armbruster wrote:
> > Michael Roth <michael.roth@amd.com> writes:
> > 
> > > Most of the current 'query-sev' command is relevant to both legacy
> > > SEV/SEV-ES guests and SEV-SNP guests, with 2 exceptions:
> > >
> > >   - 'policy' is a 64-bit field for SEV-SNP, not 32-bit, and
> > >     the meaning of the bit positions has changed
> > >   - 'handle' is not relevant to SEV-SNP
> > >
> > > To address this, this patch adds a new 'sev-type' field that can be
> > > used as a discriminator to select between SEV and SEV-SNP-specific
> > > fields/formats without breaking compatibility for existing management
> > > tools (so long as management tools that add support for launching
> > > SEV-SNP guest update their handling of query-sev appropriately).
> > 
> > Technically a compatibility break: query-sev can now return an object
> > that whose member @policy has different meaning, and also lacks @handle.
> > 
> > Matrix:
> > 
> >                             Old mgmt app    New mgmt app
> >     Old QEMU, SEV/SEV-ES       good            good(1)
> >     New QEMU, SEV/SEV-ES       good(2)         good
> >     New QEMU, SEV-SNP           bad(3)         good
> > 
> > Notes:
> > 
> > (1) As long as the management application can cope with absent member
> > @sev-type.
> > 
> > (2) As long as the management application ignores unknown member
> > @sev-type.
> > 
> > (3) Management application may choke on missing member @handle, or
> > worse, misinterpret member @policy.  Can only happen when something
> > other than the management application created the SEV-SNP guest (or the
> > user somehow made the management application create one even though it
> > doesn't know how, say with CLI option passthrough, but that's always
> > fragile, and I wouldn't worry about it here).
> > 
> > I think (1) and (2) are reasonable.  (3) is an issue for management
> > applications that support attaching to existing guests.  Thoughts?
> 
> IIUC you can only reach scenario (3) if you have created a guest
> using '-object sev-snp-guest', which is a new feature introduced
> in patch 2.
> 
> IOW, scenario (3)  old mgmt app + new QEMU + sev-snp guest does
> not exist as a combination. Thus the (bad) field is actually (n/a)
> 
> So I believe this proposed change is acceptable in all scenarios
> with existing deployed usage, as well as all newly introduced
> scenarios.

I wonder if it's worth going firther and renaming 'policy' in the 
SNP world to 'snppolicy' just to reduce the risk of accidentally
specifying the wrong one.

Dave

> Regards,
> Daniel
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-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK