[PATCH 15/16] x86/kvmclock: Stuff local APIC bus period when core crystal freq comes from CPUID
From: Sean Christopherson <seanjc@google.com>
Date: 2025-02-01 02:17:54
Also in:
kvm, linux-coco, lkml, virtualization, xen-devel
Subsystem:
kvm paravirt (kvm/paravirt), the rest, x86 architecture (32-bit and 64-bit) · Maintainers:
Paolo Bonzini, Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen
When running as a KVM guest with kvmclock support enabled, stuff the APIC timer period/frequency with the core crystal frequency from CPUID.0x15 (if CPUID.0x15 is provided). KVM's ABI adheres to Intel's SDM, which states that the APIC timer runs at the core crystal frequency when said frequency is enumerated via CPUID.0x15. The APIC timer frequency will be the processor’s bus clock or core crystal clock frequency (when TSC/core crystal clock ratio is enumerated in CPUID leaf 0x15). Signed-off-by: Sean Christopherson <seanjc@google.com> --- arch/x86/kernel/kvmclock.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 0ec867807b84..9d05d070fe25 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c@@ -106,8 +106,18 @@ static unsigned long kvm_get_tsc_khz(void) { unsigned int __tsc_khz, crystal_khz; - if (!cpuid_get_tsc_freq(&__tsc_khz, &crystal_khz)) + /* + * Prefer CPUID over kvmclock when possible, as CPUID also includes the + * core crystal frequency, i.e. the APIC timer frequency. When the core + * crystal frequency is enumerated in CPUID.0x15, KVM's ABI is that the + * (virtual) APIC BUS runs at the same frequency. + */ + if (!cpuid_get_tsc_freq(&__tsc_khz, &crystal_khz)) { +#ifdef CONFIG_X86_LOCAL_APIC + lapic_timer_period = crystal_khz * 1000 / HZ; +#endif return __tsc_khz; + } return pvclock_tsc_khz(this_cpu_pvti()); }
--
2.48.1.362.g079036d154-goog