Merge tag 'kvm-x86-pvclock-6.15' of https://github.com/kvm-x86/linux into HEAD

KVM PV clock changes for 6.15: - Don't take kvm->lock when iterating over vCPUs in the suspend notifier to fix a largely theoretical deadlock. - Use the vCPU's actual Xen PV clock information when starting the Xen timer, as the cached state in arch.hv_clock can be stale/bogus. - Fix a bug where KVM could bleed PVCLOCK_GUEST_STOPPED across different PV clocks. - Restrict PVCLOCK_GUEST_STOPPED to kvmclock, as KVM's suspend notifier only accounts for kvmclock, and there's no evidence that the flag is actually supported by Xen guests. - Clean up the per-vCPU "cache" of its reference pvclock, and instead only track the vCPU's TSC scaling (multipler+shift) metadata (which is moderately expensive to compute, and rarely changes for modern setups).
2026-05-04 19:14:18 -04:00 · 2025-03-19 09:11:59 -04:00
parent 9b093f5b86 1b3c38050b
commit fcce7c1e7d
3 changed files with 121 additions and 66 deletions
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -911,7 +911,8 @@ struct kvm_vcpu_arch {
 	int (*complete_userspace_io)(struct kvm_vcpu *vcpu);

 	gpa_t time;
-	struct pvclock_vcpu_time_info hv_clock;
+	s8  pvclock_tsc_shift;
+	u32 pvclock_tsc_mul;
 	unsigned int hw_tsc_khz;
 	struct gfn_to_pfn_cache pv_time;
 	/* set guest stopped flag in pvclock flags field */
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3121,15 +3121,17 @@ u64 get_kvmclock_ns(struct kvm *kvm)
 	return data.clock;
 }

-static void kvm_setup_guest_pvclock(struct kvm_vcpu *v,
+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 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, ref_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);
@@ -3149,52 +3151,34 @@ 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);

-	if (vcpu->pvclock_set_guest_stopped_request) {
-		vcpu->hv_clock.flags |= PVCLOCK_GUEST_STOPPED;
-		vcpu->pvclock_set_guest_stopped_request = false;
-	}
-
-	memcpy(guest_hv_clock, &vcpu->hv_clock, sizeof(*guest_hv_clock));
-
-	if (force_tsc_unstable)
-		guest_hv_clock->flags &= ~PVCLOCK_TSC_STABLE_BIT;
+	memcpy(guest_hv_clock, &hv_clock, sizeof(*guest_hv_clock));

 	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(vcpu->vcpu_id, &hv_clock);
 }

 static int kvm_guest_time_update(struct kvm_vcpu *v)
 {
+	struct pvclock_vcpu_time_info hv_clock = {};
 	unsigned long flags, tgt_tsc_khz;
 	unsigned seq;
 	struct kvm_vcpu_arch *vcpu = &v->arch;
 	struct kvm_arch *ka = &v->kvm->arch;
 	s64 kernel_ns;
 	u64 tsc_timestamp, host_tsc;
-	u8 pvclock_flags;
 	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;
@@ -3255,35 +3239,58 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)

 	if (unlikely(vcpu->hw_tsc_khz != tgt_tsc_khz)) {
 		kvm_get_time_scale(NSEC_PER_SEC, tgt_tsc_khz * 1000LL,
-				   &vcpu->hv_clock.tsc_shift,
-				   &vcpu->hv_clock.tsc_to_system_mul);
+				   &vcpu->pvclock_tsc_shift,
+				   &vcpu->pvclock_tsc_mul);
 		vcpu->hw_tsc_khz = tgt_tsc_khz;
 		kvm_xen_update_tsc_info(v);
 	}

-	vcpu->hv_clock.tsc_timestamp = tsc_timestamp;
-	vcpu->hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset;
+	hv_clock.tsc_shift = vcpu->pvclock_tsc_shift;
+	hv_clock.tsc_to_system_mul = vcpu->pvclock_tsc_mul;
+	hv_clock.tsc_timestamp = tsc_timestamp;
+	hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset;
 	vcpu->last_guest_tsc = tsc_timestamp;

 	/* If the host uses TSC clocksource, then it is stable */
-	pvclock_flags = 0;
+	hv_clock.flags = 0;
 	if (use_master_clock)
-		pvclock_flags |= PVCLOCK_TSC_STABLE_BIT;
+		hv_clock.flags |= PVCLOCK_TSC_STABLE_BIT;

-	vcpu->hv_clock.flags = pvclock_flags;
+	if (vcpu->pv_time.active) {
+		/*
+		 * GUEST_STOPPED is only supported by kvmclock, and KVM's
+		 * historic behavior is to only process the request if kvmclock
+		 * is active/enabled.
+		 */
+		if (vcpu->pvclock_set_guest_stopped_request) {
+			hv_clock.flags |= PVCLOCK_GUEST_STOPPED;
+			vcpu->pvclock_set_guest_stopped_request = 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);

-	if (vcpu->pv_time.active)
-		kvm_setup_guest_pvclock(v, &vcpu->pv_time, 0, false);
 #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(v, &vcpu->xen.vcpu_info_cache,
-					offsetof(struct compat_vcpu_info, time),
-					xen_pvclock_tsc_unstable);
+		kvm_setup_guest_pvclock(&hv_clock, v, &vcpu->xen.vcpu_info_cache,
+					offsetof(struct compat_vcpu_info, time));
 	if (vcpu->xen.vcpu_time_info_cache.active)
-		kvm_setup_guest_pvclock(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
-	kvm_hv_setup_tsc_page(v->kvm, &vcpu->hv_clock);
 	return 0;
 }

@@ -6910,23 +6917,15 @@ static int kvm_arch_suspend_notifier(struct kvm *kvm)
 {
 	struct kvm_vcpu *vcpu;
 	unsigned long i;
-	int ret = 0;

-	mutex_lock(&kvm->lock);
-	kvm_for_each_vcpu(i, vcpu, kvm) {
-		if (!vcpu->arch.pv_time.active)
-			continue;
+	/*
+	 * Ignore the return, marking the guest paused only "fails" if the vCPU
+	 * isn't using kvmclock; continuing on is correct and desirable.
+	 */
+	kvm_for_each_vcpu(i, vcpu, kvm)
+		(void)kvm_set_guest_paused(vcpu);

-		ret = kvm_set_guest_paused(vcpu);
-		if (ret) {
-			kvm_err("Failed to pause guest VCPU%d: %d\n",
-				vcpu->vcpu_id, ret);
-			break;
-		}
-	}
-	mutex_unlock(&kvm->lock);
-
-	return ret ? NOTIFY_BAD : NOTIFY_DONE;
+	return NOTIFY_DONE;
 }

 int kvm_arch_pm_notifier(struct kvm *kvm, unsigned long state)
--- a/arch/x86/kvm/xen.c
+++ b/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.pvclock_tsc_shift ||
+	    hv_clock->tsc_to_system_mul != vcpu->arch.pvclock_tsc_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.
 		 *
@@ -2261,8 +2316,8 @@ void kvm_xen_update_tsc_info(struct kvm_vcpu *vcpu)

 	entry = kvm_find_cpuid_entry_index(vcpu, function, 1);
 	if (entry) {
-		entry->ecx = vcpu->arch.hv_clock.tsc_to_system_mul;
-		entry->edx = vcpu->arch.hv_clock.tsc_shift;
+		entry->ecx = vcpu->arch.pvclock_tsc_mul;
+		entry->edx = vcpu->arch.pvclock_tsc_shift;
 	}

 	entry = kvm_find_cpuid_entry_index(vcpu, function, 2);