KVM: x86: Override TSC_STABLE flag for Xen PV clocks in kvm_guest_time_update()

When updating PV clocks, handle the Xen-specific UNSTABLE_TSC override in
the main kvm_guest_time_update() by simply clearing PVCLOCK_TSC_STABLE_BIT
in the flags of the reference pvclock structure.  Expand the comment to
(hopefully) make it obvious that Xen clocks need to be processed after all
clocks that care about the TSC_STABLE flag.

No functional change intended.

Cc: Paul Durrant <pdurrant@amazon.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://lore.kernel.org/r/20250201013827.680235-12-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/x86.c

@@ -3119,8 +3119,7 @@ u64 get_kvmclock_ns(struct kvm *kvm)
 static void kvm_setup_guest_pvclock(struct pvclock_vcpu_time_info *ref_hv_clock,
 				    struct kvm_vcpu *vcpu,
 				    struct gfn_to_pfn_cache *gpc,
-				    unsigned int offset,
-				    bool force_tsc_unstable)
+				    unsigned int offset)
 {
 	struct pvclock_vcpu_time_info *guest_hv_clock;
 	struct pvclock_vcpu_time_info hv_clock;
@@ -3155,9 +3154,6 @@ static void kvm_setup_guest_pvclock(struct pvclock_vcpu_time_info *ref_hv_clock,
 
 	memcpy(guest_hv_clock, &hv_clock, sizeof(*guest_hv_clock));
 
-	if (force_tsc_unstable)
-		guest_hv_clock->flags &= ~PVCLOCK_TSC_STABLE_BIT;
-
 	smp_wmb();
 
 	guest_hv_clock->version = ++hv_clock.version;
@@ -3178,16 +3174,6 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 	s64 kernel_ns;
 	u64 tsc_timestamp, host_tsc;
 	bool use_master_clock;
-#ifdef CONFIG_KVM_XEN
-	/*
-	 * For Xen guests we may need to override PVCLOCK_TSC_STABLE_BIT as unless
-	 * explicitly told to use TSC as its clocksource Xen will not set this bit.
-	 * This default behaviour led to bugs in some guest kernels which cause
-	 * problems if they observe PVCLOCK_TSC_STABLE_BIT in the pvclock flags.
-	 */
-	bool xen_pvclock_tsc_unstable =
-		ka->xen_hvm_config.flags & KVM_XEN_HVM_CONFIG_PVCLOCK_TSC_UNSTABLE;
-#endif
 
 	kernel_ns = 0;
 	host_tsc = 0;
@@ -3275,21 +3261,30 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 			hv_clock.flags |= PVCLOCK_GUEST_STOPPED;
 			vcpu->pvclock_set_guest_stopped_request = false;
 		}
-		kvm_setup_guest_pvclock(&hv_clock, v, &vcpu->pv_time, 0, false);
+		kvm_setup_guest_pvclock(&hv_clock, v, &vcpu->pv_time, 0);
 
 		hv_clock.flags &= ~PVCLOCK_GUEST_STOPPED;
 	}
 
 	kvm_hv_setup_tsc_page(v->kvm, &hv_clock);
 
 #ifdef CONFIG_KVM_XEN
+	/*
+	 * For Xen guests we may need to override PVCLOCK_TSC_STABLE_BIT as unless
+	 * explicitly told to use TSC as its clocksource Xen will not set this bit.
+	 * This default behaviour led to bugs in some guest kernels which cause
+	 * problems if they observe PVCLOCK_TSC_STABLE_BIT in the pvclock flags.
+	 *
+	 * Note!  Clear TSC_STABLE only for Xen clocks, i.e. the order matters!
+	 */
+	if (ka->xen_hvm_config.flags & KVM_XEN_HVM_CONFIG_PVCLOCK_TSC_UNSTABLE)
+		hv_clock.flags &= ~PVCLOCK_TSC_STABLE_BIT;
+
 	if (vcpu->xen.vcpu_info_cache.active)
 		kvm_setup_guest_pvclock(&hv_clock, v, &vcpu->xen.vcpu_info_cache,
-					offsetof(struct compat_vcpu_info, time),
-					xen_pvclock_tsc_unstable);
+					offsetof(struct compat_vcpu_info, time));
 	if (vcpu->xen.vcpu_time_info_cache.active)
-		kvm_setup_guest_pvclock(&hv_clock, v, &vcpu->xen.vcpu_time_info_cache, 0,
-					xen_pvclock_tsc_unstable);
+		kvm_setup_guest_pvclock(&hv_clock, v, &vcpu->xen.vcpu_time_info_cache, 0);
 #endif
 	return 0;
 }