Re: [PATCH] Add query-tdx-capabilities

[Markus Armbruster <armbru@redhat.com>]

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

> On Fri, Jan 09, 2026 at 11:01:27AM +0100, Markus Armbruster wrote:
>> Daniel P. Berrangé <berrange@redhat.com> writes:
>> 
>> > On Fri, Jan 09, 2026 at 10:30:32AM +0100, Markus Armbruster wrote:
>> >> Daniel P. Berrangé <berrange@redhat.com> writes:
>> >> 
>> >> > On Tue, Jan 06, 2026 at 10:36:20PM +0400, marcandre.lureau@redhat.com wrote:
>> >> >> From: Marc-André Lureau <marcandre.lureau@redhat.com>
>> >> >> 
>> >> >> Return an empty TdxCapability struct, for extensibility and matching
>> >> >> query-sev-capabilities return type.
>> >> >> 
>> >> >> Fixes: https://issues.redhat.com/browse/RHEL-129674
>> >> >> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
>> 
>> [...]
>> 
>> >> > This matches the conceptual design used with query-sev-capabilities,
>> >> > where the lack of SEV support has to be inferred from the command
>> >> > returning "GenericError".
>> >> 
>> >> Such guesswork is brittle.  An interface requiring it is flawed, and
>> >> should be improved.
>> >> 
>> >> Our SEV interface doesn't actually require it: query-sev tells you
>> >> whether we have SEV.  Just run that first.
>> >
>> > Actually these commands are intended for different use cases.
>> >
>> > "query-sev" only returns info if you have launched qemu with
>> >
>> >   $QEMU -object sev-guest,id=cgs0  -machine confidential-guest-support=cgs0
>> >
>> > The goal of "query-sev-capabilities" is to allow you to determine
>> > if the combination of host+kvm+qemu are capable of running a guest
>> > with "sev-guest".
>> >
>> > IOW, query-sev-capabilities alone is what you want/need in order
>> > to probe host features.
>> >
>> > query-sev is for examining running guest configuration
>> 
>> The doc comments fail to explain this.  Needs fixing.
>> 
>> Do management applications need to know more than "this combination of
>> host + KVM + QEMU can do SEV, yes / no?
>> 
>> If yes, what do they need?  "No" split up into serval "No, because X"?
>
> When libvirt runs  query-sev-capabilities it does not care about the
> reason for it being unsupported.   Any "GenericError" is considered
> to mark the lack of host support, and no fine grained checks are
> performed on the err msg.
>
> If query-sev-capabilities succeeds (indicating SEV is supported), then
> all the returned info is exposed to mgmt apps in the libvirt domain
> capabilities XML document.

So query-sev-capabilities is good enough as is?

If yes, then the proposed query-tdx-capabilities should also be good
enough, shouldn't it?