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 10:13:16AM -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.

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.

Regards,
Daniel
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