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

["Nikunj A. Dadhania" <nikunj@xxxxxxx>]

On 7/2/2026 1:01 AM, Sean Christopherson wrote:
> Don't override the kernel's CPU frequency calibration routine when
> registering SNP's Secure TSC calibration routine.  SNP (the architecture)
> provides zero guarantees that the CPU runs at the same frequency as the
> TSC.  The justification for clobbering the CPU routine was:
> 
>   Since the difference between CPU base and TSC frequency does not apply
>   in this case, the same callback is being used.
> 
> but that's simply not true.  E.g. if APERF/MPERF is exposed to the VM, then
> the CPU frequency absolutely does matter.
> 
> While relying on heuristics and/or the untrusted hypervisor to provide the
> CPU frequency isn't ideal, it's at least not outright wrong.
> 
> Fixes: 73bbf3b0fbba ("x86/tsc: Init the TSC for Secure TSC guests")
> Cc: Nikunj A Dadhania <nikunj@xxxxxxx>
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Nikunj A Dadhania <nikunj@xxxxxxx>

> ---
>  arch/x86/coco/sev/core.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
> index ed0ac52a765e..665de1aea0ee 100644
> --- 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;
>       x86_platform.calibrate_tsc = securetsc_get_tsc_khz;
>  
>       early_memunmap(mem, PAGE_SIZE);