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Re: [RFC PATCH v2 12/12] i386/sev: update query-sev QAPI format to handl
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From: |
Michael Roth |
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Subject: |
Re: [RFC PATCH v2 12/12] i386/sev: update query-sev QAPI format to handle SEV-SNP |
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Date: |
Fri, 3 Sep 2021 10:43:19 -0500 |
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?)
>
> Regards,
> Daniel
> --
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