[PATCH 15/16] x86/kvmclock: Stuff local APIC bus period when core crystal freq comes from CPUID

[Sean Christopherson <seanjc@google.com>]

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