[PATCH v8 5/7] kvm: x86: protect masterclock with a seqcount

[Oliver Upton <oupton@google.com>]

From: Paolo Bonzini <pbonzini@redhat.com>

Protect the reference point for kvmclock with a seqcount, so that
kvmclock updates for all vCPUs can proceed in parallel.  Xen runstate
updates will also run in parallel and not bounce the kvmclock cacheline.

nr_vcpus_matched_tsc is updated outside pvclock_update_vm_gtod_copy
though, so a spinlock must be kept for that one.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[Oliver - drop unused locals, don't double acquire tsc_write_lock]
Signed-off-by: Oliver Upton <oupton@google.com>
---
 arch/x86/include/asm/kvm_host.h |  7 ++-
 arch/x86/kvm/x86.c              | 83 +++++++++++++++++----------------
 2 files changed, 49 insertions(+), 41 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 9c34b5b63e39..5accfe7246ce 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1087,6 +1087,11 @@ struct kvm_arch {
 
 	unsigned long irq_sources_bitmap;
 	s64 kvmclock_offset;
+
+	/*
+	 * This also protects nr_vcpus_matched_tsc which is read from a
+	 * preemption-disabled region, so it must be a raw spinlock.
+	 */
 	raw_spinlock_t tsc_write_lock;
 	u64 last_tsc_nsec;
 	u64 last_tsc_write;
@@ -1097,7 +1102,7 @@ struct kvm_arch {
 	u64 cur_tsc_generation;
 	int nr_vcpus_matched_tsc;
 
-	spinlock_t pvclock_gtod_sync_lock;
+	seqcount_raw_spinlock_t pvclock_sc;
 	bool use_master_clock;
 	u64 master_kernel_ns;
 	u64 master_cycle_now;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cb5d5cad5124..29156c49cd11 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2533,9 +2533,7 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
 	vcpu->arch.this_tsc_write = kvm->arch.cur_tsc_write;
 
 	kvm_vcpu_write_tsc_offset(vcpu, offset);
-	raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
 
-	spin_lock_irqsave(&kvm->arch.pvclock_gtod_sync_lock, flags);
 	if (!matched) {
 		kvm->arch.nr_vcpus_matched_tsc = 0;
 	} else if (!already_matched) {
@@ -2543,7 +2541,7 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
 	}
 
 	kvm_track_tsc_matching(vcpu);
-	spin_unlock_irqrestore(&kvm->arch.pvclock_gtod_sync_lock, flags);
+	raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
 }
 
 static inline void adjust_tsc_offset_guest(struct kvm_vcpu *vcpu,
@@ -2731,9 +2729,6 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
 	int vclock_mode;
 	bool host_tsc_clocksource, vcpus_matched;
 
-	vcpus_matched = (ka->nr_vcpus_matched_tsc + 1 ==
-			atomic_read(&kvm->online_vcpus));
-
 	/*
 	 * If the host uses TSC clock, then passthrough TSC as stable
 	 * to the guest.
@@ -2742,6 +2737,10 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
 					&ka->master_kernel_ns,
 					&ka->master_cycle_now);
 
+	lockdep_assert_held(&kvm->arch.tsc_write_lock);
+	vcpus_matched = (ka->nr_vcpus_matched_tsc + 1 ==
+			atomic_read(&kvm->online_vcpus));
+
 	ka->use_master_clock = host_tsc_clocksource && vcpus_matched
 				&& !ka->backwards_tsc_observed
 				&& !ka->boot_vcpu_runs_old_kvmclock;
@@ -2760,14 +2759,18 @@ static void kvm_make_mclock_inprogress_request(struct kvm *kvm)
 	kvm_make_all_cpus_request(kvm, KVM_REQ_MCLOCK_INPROGRESS);
 }
 
-static void kvm_start_pvclock_update(struct kvm *kvm)
+static void __kvm_start_pvclock_update(struct kvm *kvm)
 {
-	struct kvm_arch *ka = &kvm->arch;
+	raw_spin_lock_irq(&kvm->arch.tsc_write_lock);
+	write_seqcount_begin(&kvm->arch.pvclock_sc);
+}
 
+static void kvm_start_pvclock_update(struct kvm *kvm)
+{
 	kvm_make_mclock_inprogress_request(kvm);
 
 	/* no guest entries from this point */
-	spin_lock_irq(&ka->pvclock_gtod_sync_lock);
+	__kvm_start_pvclock_update(kvm);
 }
 
 static void kvm_end_pvclock_update(struct kvm *kvm)
@@ -2776,7 +2779,8 @@ static void kvm_end_pvclock_update(struct kvm *kvm)
 	struct kvm_vcpu *vcpu;
 	int i;
 
-	spin_unlock_irq(&ka->pvclock_gtod_sync_lock);
+	write_seqcount_end(&ka->pvclock_sc);
+	raw_spin_unlock_irq(&ka->tsc_write_lock);
 	kvm_for_each_vcpu(i, vcpu, kvm)
 		kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
 
@@ -2797,20 +2801,12 @@ static void get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
 {
 	struct kvm_arch *ka = &kvm->arch;
 	struct pvclock_vcpu_time_info hv_clock;
-	unsigned long flags;
 
-	spin_lock_irqsave(&ka->pvclock_gtod_sync_lock, flags);
 	if (!ka->use_master_clock) {
-		spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
 		data->clock = get_kvmclock_base_ns() + ka->kvmclock_offset;
 		return;
 	}
 
-	data->flags |= KVM_CLOCK_TSC_STABLE;
-	hv_clock.tsc_timestamp = ka->master_cycle_now;
-	hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
-	spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
-
 	/* both __this_cpu_read() and rdtsc() should be on the same cpu */
 	get_cpu();
 
@@ -2825,6 +2821,9 @@ static void get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
 #endif
 		data->host_tsc = rdtsc();
 
+		data->flags |= KVM_CLOCK_TSC_STABLE;
+		hv_clock.tsc_timestamp = ka->master_cycle_now;
+		hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
 		kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
 				   &hv_clock.tsc_shift,
 				   &hv_clock.tsc_to_system_mul);
@@ -2839,14 +2838,14 @@ static void get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
 u64 get_kvmclock_ns(struct kvm *kvm)
 {
 	struct kvm_clock_data data;
+	struct kvm_arch *ka = &kvm->arch;
+	unsigned seq;
 
-	/*
-	 * Zero flags as it's accessed RMW, leave everything else uninitialized
-	 * as clock is always written and no other fields are consumed.
-	 */
-	data.flags = 0;
-
-	get_kvmclock(kvm, &data);
+	do {
+		seq = read_seqcount_begin(&ka->pvclock_sc);
+		data.flags = 0;
+		get_kvmclock(kvm, &data);
+	} while (read_seqcount_retry(&ka->pvclock_sc, seq));
 	return data.clock;
 }
 
@@ -2912,6 +2911,7 @@ static void kvm_setup_pvclock_page(struct kvm_vcpu *v,
 static int kvm_guest_time_update(struct kvm_vcpu *v)
 {
 	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;
@@ -2926,13 +2926,14 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 	 * If the host uses TSC clock, then passthrough TSC as stable
 	 * to the guest.
 	 */
-	spin_lock_irqsave(&ka->pvclock_gtod_sync_lock, flags);
-	use_master_clock = ka->use_master_clock;
-	if (use_master_clock) {
-		host_tsc = ka->master_cycle_now;
-		kernel_ns = ka->master_kernel_ns;
-	}
-	spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
+	seq = read_seqcount_begin(&ka->pvclock_sc);
+	do {
+		use_master_clock = ka->use_master_clock;
+		if (use_master_clock) {
+			host_tsc = ka->master_cycle_now;
+			kernel_ns = ka->master_kernel_ns;
+		}
+	} while (read_seqcount_retry(&ka->pvclock_sc, seq));
 
 	/* Keep irq disabled to prevent changes to the clock */
 	local_irq_save(flags);
@@ -5855,10 +5856,15 @@ int kvm_arch_pm_notifier(struct kvm *kvm, unsigned long state)
 
 static int kvm_vm_ioctl_get_clock(struct kvm *kvm, void __user *argp)
 {
-	struct kvm_clock_data data;
+	struct kvm_clock_data data = { 0 };
+	unsigned seq;
+
+	do {
+		seq = read_seqcount_begin(&kvm->arch.pvclock_sc);
+		data.flags = 0;
+		get_kvmclock(kvm, &data);
+	} while (read_seqcount_retry(&kvm->arch.pvclock_sc, seq));
 
-	memset(&data, 0, sizeof(data));
-	get_kvmclock(kvm, &data);
 	if (copy_to_user(argp, &data, sizeof(data)))
 		return -EFAULT;
 
@@ -8159,9 +8165,7 @@ static void kvm_hyperv_tsc_notifier(void)
 	kvm_max_guest_tsc_khz = tsc_khz;
 
 	list_for_each_entry(kvm, &vm_list, vm_list) {
-		struct kvm_arch *ka = &kvm->arch;
-
-		spin_lock_irq(&ka->pvclock_gtod_sync_lock);
+		__kvm_start_pvclock_update(kvm);
 		pvclock_update_vm_gtod_copy(kvm);
 		kvm_end_pvclock_update(kvm);
 	}
@@ -11188,8 +11192,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 
 	raw_spin_lock_init(&kvm->arch.tsc_write_lock);
 	mutex_init(&kvm->arch.apic_map_lock);
-	spin_lock_init(&kvm->arch.pvclock_gtod_sync_lock);
-
+	seqcount_raw_spinlock_init(&kvm->arch.pvclock_sc, &kvm->arch.tsc_write_lock);
 	kvm->arch.kvmclock_offset = -get_kvmclock_base_ns();
 	pvclock_update_vm_gtod_copy(kvm);
 
-- 
2.33.0.309.g3052b89438-goog