KVM: x86/xen: Use guest's copy of pvclock when starting timer

Use the guest's copy of its pvclock when starting a Xen timer, as KVM's
reference copy may not be up-to-date, i.e. may yield a false positive of
sorts.  In the unlikely scenario that the guest is starting a Xen timer
and has used a Xen pvclock in the past, but has since but turned it "off",
then vcpu->arch.hv_clock may be stale, as KVM's reference copy is updated
if and only if at least one pvclock is enabled.

Furthermore, vcpu->arch.hv_clock is currently used by three different
pvclocks: kvmclock, Xen, and Xen compat.  While it's extremely unlikely a
guest would ever enable multiple pvclocks, effectively sharing KVM's
reference clock could yield very weird behavior.  Using the guest's active
Xen pvclock instead of KVM's reference will allow dropping KVM's
reference copy.

Fixes: 451a707 ("KVM: x86/xen: improve accuracy of Xen timers")
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/20250201013827.680235-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/xen.c

@@ -150,11 +150,46 @@ static enum hrtimer_restart xen_timer_callback(struct hrtimer *timer)
 	return HRTIMER_NORESTART;
 }
 
+static int xen_get_guest_pvclock(struct kvm_vcpu *vcpu,
+				 struct pvclock_vcpu_time_info *hv_clock,
+				 struct gfn_to_pfn_cache *gpc,
+				 unsigned int offset)
+{
+	unsigned long flags;
+	int r;
+
+	read_lock_irqsave(&gpc->lock, flags);
+	while (!kvm_gpc_check(gpc, offset + sizeof(*hv_clock))) {
+		read_unlock_irqrestore(&gpc->lock, flags);
+
+		r = kvm_gpc_refresh(gpc, offset + sizeof(*hv_clock));
+		if (r)
+			return r;
+
+		read_lock_irqsave(&gpc->lock, flags);
+	}
+
+	memcpy(hv_clock, gpc->khva + offset, sizeof(*hv_clock));
+	read_unlock_irqrestore(&gpc->lock, flags);
+
+	/*
+	 * Sanity check TSC shift+multiplier to verify the guest's view of time
+	 * is more or less consistent.
+	 */
+	if (hv_clock->tsc_shift != vcpu->arch.hv_clock.tsc_shift ||
+	    hv_clock->tsc_to_system_mul != vcpu->arch.hv_clock.tsc_to_system_mul)
+		return -EINVAL;
+
+	return 0;
+}
+
 static void kvm_xen_start_timer(struct kvm_vcpu *vcpu, u64 guest_abs,
 				bool linux_wa)
 {
+	struct kvm_vcpu_xen *xen = &vcpu->arch.xen;
 	int64_t kernel_now, delta;
 	uint64_t guest_now;
+	int r = -EOPNOTSUPP;
 
 	/*
 	 * The guest provides the requested timeout in absolute nanoseconds
@@ -173,10 +208,29 @@ static void kvm_xen_start_timer(struct kvm_vcpu *vcpu, u64 guest_abs,
 	 * the absolute CLOCK_MONOTONIC time at which the timer should
 	 * fire.
 	 */
-	if (vcpu->arch.hv_clock.version && vcpu->kvm->arch.use_master_clock &&
-	    static_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
+	do {
+		struct pvclock_vcpu_time_info hv_clock;
 		uint64_t host_tsc, guest_tsc;
 
+		if (!static_cpu_has(X86_FEATURE_CONSTANT_TSC) ||
+		    !vcpu->kvm->arch.use_master_clock)
+			break;
+
+		/*
+		 * If both Xen PV clocks are active, arbitrarily try to use the
+		 * compat clock first, but also try to use the non-compat clock
+		 * if the compat clock is unusable.  The two PV clocks hold the
+		 * same information, but it's possible one (or both) is stale
+		 * and/or currently unreachable.
+		 */
+		if (xen->vcpu_info_cache.active)
+			r = xen_get_guest_pvclock(vcpu, &hv_clock, &xen->vcpu_info_cache,
+						  offsetof(struct compat_vcpu_info, time));
+		if (r && xen->vcpu_time_info_cache.active)
+			r = xen_get_guest_pvclock(vcpu, &hv_clock, &xen->vcpu_time_info_cache, 0);
+		if (r)
+			break;
+
 		if (!IS_ENABLED(CONFIG_64BIT) ||
 		    !kvm_get_monotonic_and_clockread(&kernel_now, &host_tsc)) {
 			/*
@@ -197,9 +251,10 @@ static void kvm_xen_start_timer(struct kvm_vcpu *vcpu, u64 guest_abs,
 
 		/* Calculate the guest kvmclock as the guest would do it. */
 		guest_tsc = kvm_read_l1_tsc(vcpu, host_tsc);
-		guest_now = __pvclock_read_cycles(&vcpu->arch.hv_clock,
-						  guest_tsc);
-	} else {
+		guest_now = __pvclock_read_cycles(&hv_clock, guest_tsc);
+	} while (0);
+
+	if (r) {
 		/*
 		 * Without CONSTANT_TSC, get_kvmclock_ns() is the only option.
 		 *