Re: [PATCH v3 41/41] x86/kvmclock: Get CPU base frequency from CPUID when it's available

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On Fri, 2026-05-15 at 12:19 -0700, Sean Christopherson wrote:
> If CPUID.0x16 is present and valid, use the CPU frequency provided by
> CPUID instead of assuming that the virtual CPU runs at the same
> frequency as TSC and/or kvmclock.  Back before constant TSCs were a
> thing, treating the TSC and CPU frequencies as one and the same was
> somewhat reasonable, but now it's nonsensical, especially if the
> hypervisor explicitly enumerates the CPU frequency.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kernel/kvmclock.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index 62c8ea2e6769..7607920ae386 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -190,6 +190,20 @@ void kvmclock_cpu_action(enum kvm_guest_cpu_action 
> action)
>       }
>  }
>  
> +static unsigned long kvm_get_cpu_khz(void)
> +{
> +     unsigned int cpu_khz;
> +
> +     /*
> +      * Prefer CPUID over kvmclock when possible, as the base CPU frequency
> +      * isn't necessarily the same as the kvmlock "TSC" frequency.
> +      */
> +     if (!cpuid_get_cpu_freq(&cpu_khz))
> +             return cpu_khz;
> +
> +     return pvclock_tsc_khz(this_cpu_pvti());

I'm fine with this in principle but shouldn't the fallback be calling
kvm_get_tsc_khz() instead of directly calling pvclock_tsc_khz()?

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