KVM: x86: Report host tsc and realtime values in KVM_GET_CLOCK

Handling the migration of TSCs correctly is difficult, in part because
Linux does not provide userspace with the ability to retrieve a (TSC,
realtime) clock pair for a single instant in time. In lieu of a more
convenient facility, KVM can report similar information in the kvm_clock
structure.

Provide userspace with a host TSC & realtime pair iff the realtime clock
is based on the TSC. If userspace provides KVM_SET_CLOCK with a valid
realtime value, advance the KVM clock by the amount of elapsed time. Do
not step the KVM clock backwards, though, as it is a monotonic
oscillator.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Oliver Upton <oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210916181538.968978-5-oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 33 insertions, 14 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 3ea4f6ef2474..d3631d149187 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2787,6 +2787,7 @@ static void get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
 	struct pvclock_vcpu_time_info hv_clock;
 	unsigned long flags;
 
+	data->flags = 0;
 	spin_lock_irqsave(&ka->pvclock_gtod_sync_lock, flags);
 	if (!ka->use_master_clock) {
 		spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
@@ -2803,10 +2804,20 @@ static void get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
 	get_cpu();
 
 	if (__this_cpu_read(cpu_tsc_khz)) {
+#ifdef CONFIG_X86_64
+		struct timespec64 ts;
+
+		if (kvm_get_walltime_and_clockread(&ts, &data->host_tsc)) {
+			data->realtime = ts.tv_nsec + NSEC_PER_SEC * ts.tv_sec;
+			data->flags |= KVM_CLOCK_REALTIME | KVM_CLOCK_HOST_TSC;
+		} else
+#endif
+		data->host_tsc = rdtsc();
+
 		kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
 				   &hv_clock.tsc_shift,
 				   &hv_clock.tsc_to_system_mul);
-		data->clock = __pvclock_read_cycles(&hv_clock, rdtsc());
+		data->clock = __pvclock_read_cycles(&hv_clock, data->host_tsc);
 	} else {
 		data->clock = get_kvmclock_base_ns() + ka->kvmclock_offset;
 	}
@@ -2818,12 +2829,6 @@ u64 get_kvmclock_ns(struct kvm *kvm)
 {
 	struct kvm_clock_data data;
 
-	/*
-	 * 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);
 	return data.clock;
 }
@@ -4050,7 +4055,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		r = KVM_SYNC_X86_VALID_FIELDS;
 		break;
 	case KVM_CAP_ADJUST_CLOCK:
-		r = KVM_CLOCK_TSC_STABLE;
+		r = KVM_CLOCK_VALID_FLAGS;
 		break;
 	case KVM_CAP_X86_DISABLE_EXITS:
 		r |=  KVM_X86_DISABLE_EXITS_HLT | KVM_X86_DISABLE_EXITS_PAUSE |
@@ -5847,12 +5852,16 @@ static int kvm_vm_ioctl_set_clock(struct kvm *kvm, void __user *argp)
 {
 	struct kvm_arch *ka = &kvm->arch;
 	struct kvm_clock_data data;
-	u64 now_ns;
+	u64 now_raw_ns;
 
 	if (copy_from_user(&data, argp, sizeof(data)))
 		return -EFAULT;
 
-	if (data.flags)
+	/*
+	 * Only KVM_CLOCK_REALTIME is used, but allow passing the
+	 * result of KVM_GET_CLOCK back to KVM_SET_CLOCK.
+	 */
+	if (data.flags & ~KVM_CLOCK_VALID_FLAGS)
 		return -EINVAL;
 
 	kvm_hv_invalidate_tsc_page(kvm);
@@ -5866,11 +5875,21 @@ static int kvm_vm_ioctl_set_clock(struct kvm *kvm, void __user *argp)
 	 * is slightly ahead) here we risk going negative on unsigned
 	 * 'system_time' when 'data.clock' is very small.
 	 */
-	if (kvm->arch.use_master_clock)
-		now_ns = ka->master_kernel_ns;
+	if (data.flags & KVM_CLOCK_REALTIME) {
+		u64 now_real_ns = ktime_get_real_ns();
+
+		/*
+		 * Avoid stepping the kvmclock backwards.
+		 */
+		if (now_real_ns > data.realtime)
+			data.clock += now_real_ns - data.realtime;
+	}
+
+	if (ka->use_master_clock)
+		now_raw_ns = ka->master_kernel_ns;
 	else
-		now_ns = get_kvmclock_base_ns();
-	ka->kvmclock_offset = data.clock - now_ns;
+		now_raw_ns = get_kvmclock_base_ns();
+	ka->kvmclock_offset = data.clock - now_raw_ns;
 	kvm_end_pvclock_update(kvm);
 	return 0;
 }