On Wed, Sep 01, 2021 at 04:14:10PM +0200, Markus Armbruster wrote: > Michael Roth <michael.roth@xxxxxxx> 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?
