[PATCH v3 36/41] x86/kvmclock: Get local APIC bus frequency from PV CPUID Timing Info

[Sean Christopherson <seanjc@xxxxxxxxxx>]

When running as a KVM guest with kvmclock support enabled, stuff the APIC
timer period/frequency with the local APIC bus frequency reported in
CPUID.0x40000010.EBX instead of trying to calibrate/guess the frequency.

See Documentation/virt/kvm/x86/cpuid.rst for details.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/include/asm/kvm_para.h |  1 +
 arch/x86/kernel/kvm.c           | 19 ++++++++++++++++---
 arch/x86/kernel/kvmclock.c      | 13 +++++++++++--
 3 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
index 3f7f558b5b24..381d029b72e7 100644
--- a/arch/x86/include/asm/kvm_para.h
+++ b/arch/x86/include/asm/kvm_para.h
@@ -130,6 +130,7 @@ void kvmclock_init(void);
 void kvmclock_cpu_action(enum kvm_guest_cpu_action action);
 bool kvm_para_available(void);
 unsigned int kvm_para_tsc_khz(void);
+unsigned int kvm_para_apic_bus_khz(void);
 unsigned int kvm_arch_para_features(void);
 unsigned int kvm_arch_para_hints(void);
 void kvm_async_pf_task_wait_schedule(u32 token);
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 5cd92a0b156a..bfe36e361b3c 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -918,12 +918,25 @@ bool kvm_para_available(void)
 }
 EXPORT_SYMBOL_GPL(kvm_para_available);
 
-unsigned int kvm_para_tsc_khz(void)
+static bool kvm_cpuid_has_timing_info(void)
 {
        u32 base = kvm_cpuid_base();
 
-       if (cpuid_eax(base) >= (base | KVM_CPUID_TIMING_INFO))
-               return cpuid_eax(base | KVM_CPUID_TIMING_INFO);
+       return cpuid_eax(base) >= (base | KVM_CPUID_TIMING_INFO);
+}
+
+unsigned int kvm_para_tsc_khz(void)
+{
+       if (kvm_cpuid_has_timing_info())
+               return cpuid_eax(kvm_cpuid_base() | KVM_CPUID_TIMING_INFO);
+
+       return 0;
+}
+
+unsigned int kvm_para_apic_bus_khz(void)
+{
+       if (kvm_cpuid_has_timing_info())
+               return cpuid_ebx(kvm_cpuid_base() | KVM_CPUID_TIMING_INFO);
 
        return 0;
 }
diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 5ceba4f3836c..abcc5b36ea1d 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -200,10 +200,19 @@ void kvmclock_cpu_action(enum kvm_guest_cpu_action action)
  */
 static unsigned long kvm_get_tsc_khz(void)
 {
+#ifdef CONFIG_X86_LOCAL_APIC
+       u32 apic_khz = kvm_para_apic_bus_khz();
+
        /*
-        * If KVM advertises the frequency directly in CPUID, use that
-        * instead of reverse-calculating it from the KVM clock data.
+        * Use the TSC frequency from KVM's (and other hypervisors') PV CPUID
+        * leaf when available, instead of reverse-calculating it from the KVM
+        * clock data.  As a bonus, the CPUID leaf also includes the local APIC
+        * bus/timer frequency.
         */
+       if (apic_khz)
+               lapic_timer_period = apic_khz;
+#endif
+
        return kvm_para_tsc_khz() ? : pvclock_tsc_khz(this_cpu_pvti());
 }
 
-- 
2.54.0.563.g4f69b47b94-goog