[v2,36/38] x86/kvmclock: Stuff local APIC bus period when core crystal freq comes from CPUID

Commit Message

Sean Christopherson Feb. 27, 2025, 2:18 a.m. UTC

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 <[e-mail]>
---
 arch/x86/kernel/kvmclock.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

Patch

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 3efb837c7406..80d9c86e0671 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -192,8 +192,18 @@  static unsigned long kvm_get_tsc_khz(void)
 {
 	struct cpuid_tsc_info info;
 
-	if (!cpuid_get_tsc_freq(&info))
+	/*
+	 * 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(&info)) {
+#ifdef CONFIG_X86_LOCAL_APIC
+		lapic_timer_period = info.crystal_khz * 1000 / HZ;
+#endif
 		return info.tsc_khz;
+	}
 
 	return pvclock_tsc_khz(this_cpu_pvti());
 }