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? 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. 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 :|
