[PATCH v2 34/38] x86/kvmclock: 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/kvmclock.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index b924b19e8f0f..c45b321533e5 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -188,6 +188,20 @@ void kvmclock_cpu_action(enum kvm_guest_cpu_action action)
 	}
 }
 
+static unsigned long kvm_get_cpu_khz(void)
+{
+	unsigned int cpu_khz;
+
+	/*
+	 * Prefer CPUID over kvmclock when possible, as the base CPU frequency
+	 * isn't necessarily the same as the kvmlock "TSC" frequency.
+	 */
+	if (!cpuid_get_cpu_freq(&cpu_khz))
+		return cpu_khz;
+
+	return pvclock_tsc_khz(this_cpu_pvti());
+}
+
 /*
  * If we don't do that, there is the possibility that the guest
  * will calibrate under heavy load - thus, getting a lower lpj -
@@ -418,7 +432,7 @@ void __init kvmclock_init(void)
 
 	kvm_sched_clock_init(stable);
 
-	tsc_register_calibration_routines(kvm_get_tsc_khz, kvm_get_tsc_khz,
+	tsc_register_calibration_routines(kvm_get_tsc_khz, kvm_get_cpu_khz,
 					  tsc_properties);
 
 	x86_platform.get_wallclock = kvm_get_wallclock;
-- 
2.48.1.711.g2feabab25a-goog