[PATCH v5 24/51] x86/kvm: Get CPU base frequency from CPUID when it's available
From: Sean Christopherson <seanjc@google.com>
Date: 2026-07-01 19:32:56
Also in:
kvm, linux-coco, linux-doc, 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
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@google.com> --- arch/x86/kernel/kvm.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+)
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 1cef54e1e7d9..6c7011ff7bd1 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c@@ -50,6 +50,7 @@ #include <asm/e820/api.h> static unsigned int kvm_tsc_khz_cpuid __initdata; +static unsigned int kvm_cpu_khz_cpuid __initdata; DEFINE_STATIC_KEY_FALSE_RO(kvm_async_pf_enabled);
@@ -928,6 +929,11 @@ static unsigned int __init kvm_get_tsc_khz(void) return kvm_tsc_khz_cpuid; } +static unsigned int __init kvm_get_cpu_khz(void) +{ + return kvm_cpu_khz_cpuid; +} + unsigned int kvm_arch_para_features(void) { return cpuid_eax(kvm_cpuid_base() | KVM_CPUID_FEATURES);
@@ -1041,6 +1047,14 @@ static void __init kvm_init_platform(void) } } + /* + * Prefer CPUID.0x16 over KVM's PV CPUID when possible, as the base CPU + * frequency isn't necessarily the same as the TSC frequency. + */ + kvm_cpu_khz_cpuid = __cpu_khz_from_cpuid(); + if (kvm_cpu_khz_cpuid) + x86_init.hyper.get_cpu_khz = kvm_get_cpu_khz; + /* * If the TSC counts at a constant frequency across P/T states and in * deep C-states, treat the TSC reliable, as guaranteed by KVM.
--
2.55.0.rc0.799.gd6f94ed593-goog