Re: [PATCH 10/11] x86/sev: Extend the config-fs attestation support for an SVSM

[Dan Williams]

Tom Lendacky wrote:
[..]
> > If it is the same format, and the input is user controlled then I am
> > confused what the new ABI is selecting? Could it not be inferred by
> > privlevel?
> 
> The new ABI selects whether to go through the SVSM to get the attestation
> report, which will additionally return a manifest that, along with the
> nonce, has become part of the report through hashing.

Ah yeah, that's a lot to overlead to the meaning of privlevel.

> 
> But, yes, I mentioned in a previous reply [1] that we could use privlevel
> to determine whether to invoke attestation through an SVSM request or
> through the standard method of issuing an extended guest request.
> 
> [1] https://lore.kernel.org/lkml/3fca61f2-6fe0-4431-818e-9c7b96c6a391@xxxxxxx/

Missed that, thanks. Lets keep explicit as you have it and not overload
privlevel.

[..]
> > Can the version be made to not matter, or be inferred by the presence of
> > a new enumerated service capability? For example, make the same compat
> > guarantees that ACPI methods do between versions where extensions are
> > optional and software can always use v1 without breaking? Otherwise, I
> > am not grokking what software should do with this version.
> 
> Software can always use v1. The idea is that if a service wanted to
> provide additional information or change the information provided in the
> service manifest, then it would have to do that via a new version of its
> manifest so as not to break existing users. By default, zero would be used
> for the service manifest version and have to be updated by the user if
> they wanted a different one.

Can it just be the case the manifest can only grow but old fields never
change? Then software can determine the "version" based on manifest size
and no software gets built with an explicit version check, and is
instead built to understand a certain point in the evolution of the
manifest.

To be clear this is my standard response to any specification that
transmits a payload that "may change in the future", if it is an
awkward fit in this case it would at least be good to clarify why.

> > Separately, is this a version for the service protocol or a version of
> > the manifest format? The description makes it sound like the latter, but
> > the "service_version" name makes it sound like the former.
> 
> Correct, it is for the manifest version. I can rename it to
> service_manifest or service_manifest_version. I'd rather not rename it to
> manifest_version since it is specific to an individual service.

FWIW, service_manifest_version makes it crystal clear to me, but maybe
even better would be that the output size already conveys that, this
attribute is not needed, and userspace reads as much of the manifest as
it knows about.