[PATCH v4 30/30] x86/xen: Obtain TSC frequency from CPUID if present

[David Woodhouse <dwmw2@infradead.org>]

From: David Woodhouse <dwmw@amazon.co.uk>

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@amazon.co.uk>
---
 arch/x86/kvm/x86.c  |  3 +--
 arch/x86/xen/time.c | 12 ++++++++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c15303963686..ac982652e5e0 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3469,7 +3469,6 @@ static void kvm_setup_guest_pvclock(struct pvclock_vcpu_time_info *ref_hv_clock,
 int kvm_guest_time_update(struct kvm_vcpu *v)
 {
 	struct pvclock_vcpu_time_info hv_clock = {};
-	unsigned long flags;
 	u64 tgt_tsc_hz;
 	unsigned seq;
 	struct kvm_vcpu_arch *vcpu = &v->arch;
@@ -10162,7 +10161,7 @@ static void kvm_hyperv_tsc_notifier(void)
 	kvm_caps.max_guest_tsc_khz = tsc_khz;
 
 	list_for_each_entry(kvm, &vm_list, vm_list) {
-		__kvm_start_pvclock_update(kvm);
+		__kvm_start_pvclock_update(kvm, NULL);
 		pvclock_update_vm_gtod_copy(kvm);
 		kvm_end_pvclock_update(kvm);
 	}
diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
index 6f9f665bb7ae..862b8e9e8405 100644
--- a/arch/x86/xen/time.c
+++ b/arch/x86/xen/time.c
@@ -41,8 +41,20 @@ static unsigned long xen_tsc_khz(void)
 {
 	struct pvclock_vcpu_time_info *info =
 		&HYPERVISOR_shared_info->vcpu_info[0].time;
+	u32 eax, ebx, ecx, edx;
+	u32 base = xen_cpuid_base();
 
 	setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
+
+	/*
+	 * 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);
 }
 
-- 
2.51.0