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

[Thomas Gleixner]

On Tue, Aug 29 2023 at 16:01, Jun Nakajima wrote:
>> On Aug 25, 2023, at 10:09 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>>> The newer spec says "Virtual TSC values are consistent among all the TD’s
>>> VCPUs at the level supported by the CPU".
>> 
>> That means what? It's not a guarantee for consistency either. :(
>
> Actually (in TDX Module 1.5 spec), the sentence is "Virtual TSC values
> are consistent among all the TD’s VCPUs at the level supported by the
> CPU, see below”.
>
> And the below:
> ---
> The host VMM is required to do the following:
> • Set up the same IA32_TSC_ADJUST values on all LPs before initializing the Intel TDX module.
> • Make sure IA32_TSC_ADJUST is not modified from its initial value before calling SEAMCALL.
>
> The Intel TDX module checks the above as part of TDH.VP.ENTER and any
> other SEAMCALL leaf function that reads TSC.

What happens when the check detects that the host modified TSC ADJUST?

What validates the VMCS TSC offset field?

> The virtualized TSC is designed to have the following characteristics:
> • The virtual TSC frequency is specified by the host VMM as an input
> to TDH.MNG.INIT in units of 25MHz – it can be between 4 and 400
> (corresponding to a range of 100MHz to 10GHz).

What validates that the frequency is correct?

How is ensured that the host does not change TSC scaling?

Thanks,

        tglx