[PATCH v10 02/21] KVM: x86: Improve accuracy of KVM clock when TSC scaling is in force
From: Sean Christopherson
Date: Wed Aug 26 2026 - 17:33:25 EST
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.
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 683bb8bf43a9..0255ca6b24c2 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;
struct gfn_to_pfn_cache pv_time;
/* set guest stopped flag in pvclock flags field */
bool pvclock_set_guest_stopped_request;
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index ddb022cb203a..ca26be88c204 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -16,6 +16,7 @@
#include <linux/vmalloc.h>
#include <linux/uaccess.h>
#include <linux/sched/stat.h>
+#include <linux/units.h>
#include <asm/processor.h>
#include <asm/user.h>
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 10f11e06d117..e18158f51103 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -61,6 +61,7 @@
#include <linux/mem_encrypt.h>
#include <linux/suspend.h>
#include <linux/smp.h>
+#include <linux/units.h>
#include <trace/events/ipi.h>
#include <trace/events/kvm.h>
@@ -1763,7 +1764,8 @@ static void kvm_setup_guest_pvclock(struct pvclock_vcpu_time_info *ref_hv_clock,
int kvm_guest_time_update(struct kvm_vcpu *v)
{
struct pvclock_vcpu_time_info hv_clock = {};
- unsigned long flags, tgt_tsc_khz;
+ unsigned long flags;
+ u64 tgt_tsc_hz;
unsigned seq;
struct kvm_vcpu_arch *vcpu = &v->arch;
struct kvm_arch *ka = &v->kvm->arch;
@@ -1789,8 +1791,8 @@ int kvm_guest_time_update(struct kvm_vcpu *v)
/* Keep irq disabled to prevent changes to the clock */
local_irq_save(flags);
- tgt_tsc_khz = get_cpu_tsc_khz();
- if (unlikely(tgt_tsc_khz == 0)) {
+ tgt_tsc_hz = (u64)get_cpu_tsc_khz() * HZ_PER_KHZ;
+ if (unlikely(tgt_tsc_hz == 0)) {
local_irq_restore(flags);
kvm_make_request(KVM_REQ_CLOCK_UPDATE, v);
return 1;
@@ -1831,16 +1833,16 @@ int kvm_guest_time_update(struct kvm_vcpu *v)
/* With all the info we got, fill in the values */
if (kvm_caps.has_tsc_control) {
- tgt_tsc_khz = kvm_scale_tsc(tgt_tsc_khz,
+ tgt_tsc_hz = kvm_scale_tsc(tgt_tsc_hz,
v->arch.l1_tsc_scaling_ratio);
- tgt_tsc_khz = tgt_tsc_khz ? : 1;
+ tgt_tsc_hz = tgt_tsc_hz ? : 1;
}
- if (unlikely(vcpu->hw_tsc_khz != tgt_tsc_khz)) {
- kvm_get_time_scale(NSEC_PER_SEC, tgt_tsc_khz * 1000LL,
+ if (unlikely(vcpu->hw_tsc_hz != tgt_tsc_hz)) {
+ kvm_get_time_scale(NSEC_PER_SEC, tgt_tsc_hz,
&vcpu->pvclock_tsc_shift,
&vcpu->pvclock_tsc_mul);
- vcpu->hw_tsc_khz = tgt_tsc_khz;
+ vcpu->hw_tsc_hz = tgt_tsc_hz;
}
hv_clock.tsc_shift = vcpu->pvclock_tsc_shift;
--
2.55.0.887.g758fc8c411-goog