[PATCH v4 20/47] x86/kvm: Get CPU base frequency from CPUID when it's available

[Sean Christopherson]

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/kvm.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index c1139182121d..c81a24d0efdf 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);
@@ -1049,6 +1055,14 @@ static void __init kvm_init_platform(void)
 #endif
 	}
 
+	/*
+	 * 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, counts
          * in deep C-states, and the TSC hasn't been marked unstable, treat the
-- 
2.54.0.823.g6e5bcc1fc9-goog