Re: [PATCH v9 02/21] KVM: x86: Improve accuracy of KVM clock when TSC scaling is in force

[Sean Christopherson]

On Mon, Aug 10, 2026, Sean Christopherson wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
> 
> The kvm_guest_time_update() function scales the host TSC frequency to
> the guest's using kvm_scale_tsc() and the v->arch.l1_tsc_scaling_ratio
> scaling ratio previously calculated for that vCPU. Then calculates the
> scaling factors for the KVM clock itself based on that guest TSC
> frequency.
> 
> However, it uses kHz as the unit when scaling, and then multiplies by
> 1000 only at the end.
> 
> With a host TSC frequency of 3000MHz and a guest set to 2500MHz, the
> result of kvm_scale_tsc() will actually come out at 2,499,999kHz. So
> the KVM clock advertised to the guest is based on a frequency of
> 2,499,999,000 Hz.
> 
> By using Hz as the unit from the beginning, the KVM clock would be based
> on a more accurate frequency of 2,499,999,999 Hz in this example.
> 
> Use u64 for the hw_tsc_hz field since an unsigned int would overflow for
> TSC frequencies above 4GHz. Use div_u64() for the Xen CPUID leaf to
> play nice with 32-bit kernels.
> 
> Fixes: 78db6a503796 ("KVM: x86: rewrite handling of scaled TSC for kvmclock")
> Reviewed-by: Paul Durrant <paul@xxxxxxx>
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> Tested-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  2 +-
>  arch/x86/kvm/cpuid.c            |  1 +
>  arch/x86/kvm/x86.c              | 18 ++++++++++--------
>  3 files changed, 12 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 283847619ff8..6ddc988576d0 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -864,7 +864,7 @@ struct kvm_vcpu_arch {
>  	gpa_t time;
>  	s8  pvclock_tsc_shift;
>  	u32 pvclock_tsc_mul;
> -	unsigned int hw_tsc_khz;
> +	u64 hw_tsc_hz;

>From https://sashiko.dev/#/patchset/20260810225500.869288-1-seanjc%40google.com:

 : > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
 : > index b517257a63153..099c3da20547d 100644
 : > --- a/arch/x86/include/asm/kvm_host.h
 : > +++ b/arch/x86/include/asm/kvm_host.h
 : > @@ -1004,7 +1004,7 @@ struct kvm_vcpu_arch {
 : >  	gpa_t time;
 : >  	s8  pvclock_tsc_shift;
 : >  	u32 pvclock_tsc_mul;
 : > -	unsigned int hw_tsc_khz;
 : > +	u64 hw_tsc_hz;
 : >  	struct gfn_to_pfn_cache pv_time;
 : >  	/* set guest stopped flag in pvclock flags field */
 : >  	bool pvclock_set_guest_stopped_request;
 : 
 : The commit message mentions using div_u64() for the Xen CPUID leaf, but is
 : the actual update to kvm_cpuid() missing here?
 : 
 : It appears kvm_cpuid() still references the old field name:
 : 
 : arch/x86/kvm/cpuid.c:kvm_cpuid() {
 :     ...
 :     *eax = vcpu->arch.hw_tsc_khz;
 :     ...
 : }
 : 
 : Since the field was renamed to hw_tsc_hz in struct kvm_vcpu_arch, will this
 : cause a build failure when compiling for x86?

The changelog is stale.  This will be applied on top of commit 7d3bd21e457b
("KVM: x86: Remove runtime Xen TSC frequency CPUID update") that's sitting in
kvm-x86/clocks.  I'll drop the stale sentence from the changelog when applying.