[PATCH v3 35/41] x86/kvmclock: Obtain TSC frequency from CPUID if present

[Sean Christopherson <seanjc@xxxxxxxxxx>]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

In https://lkml.org/lkml/2008/10/1/246 a proposal was made for generic
CPUID conventions across hypervisors. It was mostly shot down in flames,
but the leaf at 0x40000010 containing timing information didn't die.

It's used by XNU and FreeBSD guests under all hypervisors¹² to determine
the TSC frequency, and also exposed by the EC2 Nitro hypervisor (as
well as, presumably, VMware). FreeBSD's Bhyve is probably just about
to start exposing it too.

Use it under KVM to obtain the TSC frequency more accurately, instead
of reverse-calculating the frequency from the mul/shift values in the
KVM clock.

Before:
[    0.000020] tsc: Detected 2900.014 MHz processor

After:
[    0.000020] tsc: Detected 2900.015 MHz processor

$ cpuid -1 -l 0x40000010
CPU:
   hypervisor generic timing information (0x40000010):
      TSC frequency (Hz) = 2900015
      bus frequency (Hz) = 1000000

¹ https://github.com/apple/darwin-xnu/blob/main/osfmk/i386/cpuid.c
² https://github.com/freebsd/freebsd-src/commit/4a432614f68

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/include/asm/kvm_para.h |  1 +
 arch/x86/kernel/kvm.c           | 10 ++++++++++
 arch/x86/kernel/kvmclock.c      |  6 +++++-
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
index 17053d2bf270..3f7f558b5b24 100644
--- a/arch/x86/include/asm/kvm_para.h
+++ b/arch/x86/include/asm/kvm_para.h
@@ -129,6 +129,7 @@ enum kvm_guest_cpu_action {
 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_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 65c787b1ea03..5cd92a0b156a 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -918,6 +918,16 @@ bool kvm_para_available(void)
 }
 EXPORT_SYMBOL_GPL(kvm_para_available);
 
+unsigned int kvm_para_tsc_khz(void)
+{
+       u32 base = kvm_cpuid_base();
+
+       if (cpuid_eax(base) >= (base | KVM_CPUID_TIMING_INFO))
+               return cpuid_eax(base | KVM_CPUID_TIMING_INFO);
+
+       return 0;
+}
+
 unsigned int kvm_arch_para_features(void)
 {
        return cpuid_eax(kvm_cpuid_base() | KVM_CPUID_FEATURES);
diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 47f7df1e81a0..5ceba4f3836c 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -200,7 +200,11 @@ void kvmclock_cpu_action(enum kvm_guest_cpu_action action)
  */
 static unsigned long kvm_get_tsc_khz(void)
 {
-       return pvclock_tsc_khz(this_cpu_pvti());
+       /*
+        * If KVM advertises the frequency directly in CPUID, use that
+        * instead of reverse-calculating it from the KVM clock data.
+        */
+       return kvm_para_tsc_khz() ? : pvclock_tsc_khz(this_cpu_pvti());
 }
 
 static void __init kvm_get_preset_lpj(void)
-- 
2.54.0.563.g4f69b47b94-goog