Re: [PATCH v6 06/51] x86/sev: Don't override CPU frequency calibration for SNP's Secure TSC

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On Thu, 2026-08-06 at 16:35 -0700, 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>
> Reviewed-by: Nikunj A Dadhania <nikunj@xxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: David Woodhouse <dwmw@xxxxxxxxxxxx>

I don't think I care about Sashiko's complaint. Even if we still clamp
it in generic code, I think that's better than having those in the
platform-specific code.

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