[PATCH v4 21/47] x86/xen: Obtain TSC frequency from CPUID if present

[Sean Christopherson <seanjc@xxxxxxxxxx>]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

The Xen CPUID leaf 3, sub-leaf 0, ECX provides the guest TSC frequency
in kHz directly. Use it when available instead of reverse-calculating
the frequency from the pvclock tsc_to_system_mul and tsc_shift values,
which loses precision.

This mirrors the equivalent change for KVM guests using the generic
0x40000010 timing leaf.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
[sean: drop non-Xen changes]
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/xen/time.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
index 487ad838c441..36d66abf5379 100644
--- a/arch/x86/xen/time.c
+++ b/arch/x86/xen/time.c
@@ -42,6 +42,17 @@ static unsigned int __init xen_tsc_khz(void)
 {
        struct pvclock_vcpu_time_info *info =
                &HYPERVISOR_shared_info->vcpu_info[0].time;
+       u32 base = xen_cpuid_base();
+       u32 eax, ebx, ecx, edx;
+
+       /*
+        * If Xen provides the guest TSC frequency directly in CPUID
+        * (leaf 3, sub-leaf 0, ECX), use that instead of reverse-
+        * calculating from the pvclock mul/shift.
+        */
+       cpuid_count(base + 3, 0, &eax, &ebx, &ecx, &edx);
+       if (ecx)
+               return ecx;
 
        return pvclock_tsc_khz(info);
 }
-- 
2.54.0.823.g6e5bcc1fc9-goog