Re: [PATCH v5 06/51] x86/sev: Don't override CPU frequency calibration for SNP's Secure TSC

On Wed, Jul 01, 2026, [email protected] wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] The patch removes the `calibrate_cpu` override to allow the CPU base frequency to differ from the TSC frequency, but fails to account for `determine_cpu_tsc_frequencies()` which will forcefully clobber `cpu_khz` if it diverges by more than 10%.
> --
> 
> commit 6d3b9e81661ead5f25434666266aec6514dadaa8
> Author: Sean Christopherson <[email protected]>
> 
> x86/sev: Don't override CPU frequency calibration for SNP's Secure TSC
> 
> This commit removes the override of the kernel's CPU frequency calibration
> routine when registering SNP's Secure TSC. It aims to decouple the CPU base
> frequency from the TSC frequency, as exposing APERF/MPERF requires an
> accurate CPU frequency.
> 
> > diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
> > --- a/arch/x86/coco/sev/core.c
> > +++ b/arch/x86/coco/sev/core.c
> > @@ -2046,7 +2046,6 @@ void __init snp_secure_tsc_init(void)
> >  
> >  	snp_tsc_freq_khz = SNP_SCALE_TSC_FREQ(tsc_freq_mhz * 1000, secrets->tsc_factor);
> >  
> > -	x86_platform.calibrate_cpu = securetsc_get_tsc_khz;
> 
> [Severity: Medium]
> Will this removal be sufficient to decouple cpu_khz from tsc_khz if they
> diverge significantly?

No, but IMO it's a step in the right direction.

> Looking at determine_cpu_tsc_frequencies() in arch/x86/kernel/tsc.c, there
> is a generic sanity check that enforces a 10% limit on the difference
> between the two frequencies:
> 
> arch/x86/kernel/tsc.c:determine_cpu_tsc_frequencies() {
>     ...
>     else if (abs(cpu_khz - tsc_khz) * 10 > tsc_khz)
>         cpu_khz = tsc_khz;
>     ...
> }
> 
> If the true host CPU frequency diverges from the guest TSC frequency by more
> than 10%, won't this logic forcefully overwrite the correct cpu_khz back to
> tsc_khz, unintentionally undoing the fix for the APERF/MPERF calculation?

Probably, but this series is already boiling a pretty large lake, I think the
ocean can be left for a future cleanup.