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

[Markus Armbruster]

Daniel P. Berrangé <berrange@redhat.com> writes:

> 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?
>
> 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.

Let's work this into the commit message.