Re: [RFC 0/3] Enlightened vTPM support for SVSM on SEV-SNP

[James Bottomley <jejb@linux.ibm.com>]

On Wed, 2023-01-04 at 16:44 -0600, Tom Lendacky wrote:
> How should we indicate when cancellation or locality support is
> available? Should there just be a protocol call ID that returns a
> bitmap of supported command values (e.g.: bit 8 would be set for
> TPM_SEND_COMMAND) and a bitmap of supported features?

I'm not sure anyone will ever care about cancellation.  There are only
two cancellable points in the entire MS TPM implementation; one is in
CryptRsa to abort the prime search and the other is in
CryptEccCommitCompute.  On a full power CPU system you're unlikely ever
to be able to signal a cancellation in time for it to be meaningfully
useful.

Discovery for all the commands bar locality is simply they return -
EINVAL if you try to invoke them in the SVSM when they're not
implemented (that's an extension to the MSSIM protocol).

Locality can't simply be added, it needs someone to come up with the
actual locality properties and for the SVSM to police them.  Presumably
discovery would be part of that exercise.

James