Re: [PATCH] x86/tdx: Mark TSC reliable

[Thomas Gleixner]

On Tue, Aug 08 2023 at 23:01, Kirill A. Shutemov wrote:
> On Tue, Aug 08, 2023 at 10:13:05AM -0700, Dave Hansen wrote:
>> I take it this is carved in stone in the TDX specs somewhere.  A
>> reference would be nice.
>
> TDX Module 1.0 spec:
>
> 	5.3.5. Time Stamp Counter (TSC)
>
> 	TDX provides a trusted virtual TSC to the guest TDs. TSC value is
> 	monotonously incrementing, starting from 0 on TD initialization by the
> 	host VMM. The deviation between virtual TSC values read by each VCPU is
> 	small.

Nice weasel wording. What's the definition of "small"?

Any OS needs a guarantee that vCPUs cannot observe time going backwards,
which is obviously possible when the deviation is not small enough.

> Wording in the spec looks okay to me. We can only hope that implementation
> going to be sane.

Hope dies last :)