Re: [PATCH v15 04/13] x86/sev: Change TSC MSR behavior for Secure TSC enabled guests

Borislav Petkov
2024-12-11T22:23:21Z

On Wed, Dec 11, 2024 at 04:01:31PM -0600, Tom Lendacky wrote:
> It could be any reason... maybe the hypervisor wants to know when this
> MSR used in order to tell the guest owner to update their code. Writing
> to or reading from that MSR is not that common, so I would think we want
> to keep the same behavior that has been in effect.

Ah, I thought you're gonna say something along the lines of, yeah, we must use
the HV GHCB protocol because of <raisins> and there's no other way this could
work.

> But if we do want to make this change, maybe do it separate from the
> Secure TSC series since it alters the behavior of SEV-ES guests and
> SEV-SNP guests without Secure TSC.

Already suggested so - this should be a separate patch.

It would be interesting to see if it brings any improvement by avoiding the HV
call... especially since RDTSC is used a *lot* and prominently at that in
sched_clock, for example.

--
Regards/Gruss,
Boris.