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

[Daniel P . Berrangé]

On Fri, Sep 03, 2021 at 10:43:19AM -0500, Michael Roth wrote:
> On Fri, Sep 03, 2021 at 04:30:48PM +0100, Daniel P. Berrangé wrote:
> > On Fri, Sep 03, 2021 at 10:13:16AM -0500, Michael Roth 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?
> > > 
> > > Hmm... yah I hadn't considering 'old mgmt' trying to interact with a SNP
> > > guest started through some other means.
> > > 
> > > Don't really see an alternative other than introducing a new
> > > 'query-sev-snp', but that would still leave 'old mgmt' broken, since
> > > it might still call do weird stuff like try to interpret the SNP policy
> > > as an SEV/SEV-ES and end up with some very unexpected results. So if I
> > > did go this route, I would need to have QMP begin returning an error if
> > > query-sev is run against an SNP guest. But currently for non-SEV guests
> > > it already does:
> > > 
> > >   error_setg(errp, "SEV feature is not available")
> > > 
> > > so 'old mgmt' should be able to handle the error just fine.
> > > 
> > > Would that approach be reasonable?
> > 
> > This ties into the question I've just sent in my other mail.
> > 
> > If the hardware strictly requires that guest are created in SEV-SNP
> > mode only, and will not support SEV/SEV-ES mode, then we need to
> > ensure "query-sev" reports the feature as not-available, so that
> > existing mgmt apps don't try to use SEV/SEV-ES.
> 
> An SEV-SNP-capable host can run both 'legacy' SEV/SEV-ES, as well as
> SEV-SNP guests, at the same time. But as far as QEMU goes, we need
> to specify one or the other explicitly at launch time, via existing
> 'sev-guest', or the new 'sev-snp-guest' ConfidentialGuestSupport type.
> 
> > 
> > If the SEV-SNP hardware is functionally back-compatible with a gues
> > configured in SEV/SEV-ES mode, then we souldn't need a new command,
> > just augment th eexisting command with additional field(s), to
> > indicate existance of SEV-SNP features.
> 
> query-sev-info provides information specific to the guest instance,
> like the configured policy. Are you thinking of query-sev-capabilities,
> which reports some host-wide information and should indeed remain
> available for either case. (and maybe should also be updated to report
> on SEV-SNP availability for the host?)

Oh right, yes, I am getting confused with query-sev-capabilities.

I think this means everything is OK with your query-sev-info
changes proposed here, I'll reply direct to Markus' point though.

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|