[PATCH 14/16] x86/kvmclock: Get TSC frequency from CPUID when its available

[Sean Christopherson <seanjc@google.com>]

When kvmclock and CPUID.0x15 are both present, use the TSC frequency from
CPUID.0x15 instead of kvmclock's frequency.  Barring a misconfigured
setup, both sources should provide the same frequency, CPUID.0x15 is
arguably a better source when using the TSC over kvmclock, and most
importantly, using CPUID.0x15 will allow stuffing the local APIC timer
frequency based on the core crystal frequency, i.e. will allow skipping
APIC timer calibration.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kernel/kvmclock.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 66e53b15dd1d..0ec867807b84 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -102,6 +102,16 @@ static inline void kvm_sched_clock_init(bool stable)
 		sizeof(((struct pvclock_vcpu_time_info *)NULL)->system_time));
 }
 
+static unsigned long kvm_get_tsc_khz(void)
+{
+	unsigned int __tsc_khz, crystal_khz;
+
+	if (!cpuid_get_tsc_freq(&__tsc_khz, &crystal_khz))
+		return __tsc_khz;
+
+	return pvclock_tsc_khz(this_cpu_pvti());
+}
+
 static unsigned long kvm_get_cpu_khz(void)
 {
 	unsigned int cpu_khz;
@@ -125,11 +135,6 @@ static unsigned long kvm_get_cpu_khz(void)
  * poll of guests can be running and trouble each other. So we preset
  * lpj here
  */
-static unsigned long kvm_get_tsc_khz(void)
-{
-	return pvclock_tsc_khz(this_cpu_pvti());
-}
-
 static void __init kvm_get_preset_lpj(void)
 {
 	unsigned long khz;
-- 
2.48.1.362.g079036d154-goog