KVM: x86: Don't bleed PVCLOCK_GUEST_STOPPED across PV clocks

When updating a specific PV clock, make a full copy of KVM's reference
copy/cache so that PVCLOCK_GUEST_STOPPED doesn't bleed across clocks.
E.g. in the unlikely scenario the guest has enabled both kvmclock and Xen
PV clock, a dangling GUEST_STOPPED in kvmclock would bleed into Xen PV
clock.

Using a local copy of the pvclock structure also sets the stage for
eliminating the per-vCPU copy/cache (only the TSC frequency information
actually "needs" to be cached/persisted).

Fixes: aa096aa ("KVM: x86/xen: setup pvclock updates")
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://lore.kernel.org/r/20250201013827.680235-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/x86.c

@@ -3123,8 +3123,11 @@ static void kvm_setup_guest_pvclock(struct kvm_vcpu *v,
 {
 	struct kvm_vcpu_arch *vcpu = &v->arch;
 	struct pvclock_vcpu_time_info *guest_hv_clock;
+	struct pvclock_vcpu_time_info hv_clock;
 	unsigned long flags;
 
+	memcpy(&hv_clock, &vcpu->hv_clock, sizeof(hv_clock));
+
 	read_lock_irqsave(&gpc->lock, flags);
 	while (!kvm_gpc_check(gpc, offset + sizeof(*guest_hv_clock))) {
 		read_unlock_irqrestore(&gpc->lock, flags);
@@ -3144,25 +3147,25 @@ static void kvm_setup_guest_pvclock(struct kvm_vcpu *v,
 	 * it is consistent.
 	 */
 
-	guest_hv_clock->version = vcpu->hv_clock.version = (guest_hv_clock->version + 1) | 1;
+	guest_hv_clock->version = hv_clock.version = (guest_hv_clock->version + 1) | 1;
 	smp_wmb();
 
 	/* retain PVCLOCK_GUEST_STOPPED if set in guest copy */
-	vcpu->hv_clock.flags |= (guest_hv_clock->flags & PVCLOCK_GUEST_STOPPED);
+	hv_clock.flags |= (guest_hv_clock->flags & PVCLOCK_GUEST_STOPPED);
 
-	memcpy(guest_hv_clock, &vcpu->hv_clock, sizeof(*guest_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 = ++vcpu->hv_clock.version;
+	guest_hv_clock->version = ++hv_clock.version;
 
 	kvm_gpc_mark_dirty_in_slot(gpc);
 	read_unlock_irqrestore(&gpc->lock, flags);
 
-	trace_kvm_pvclock_update(v->vcpu_id, &vcpu->hv_clock);
+	trace_kvm_pvclock_update(v->vcpu_id, &hv_clock);
 }
 
 static int kvm_guest_time_update(struct kvm_vcpu *v)