Revert "x86/kvm: On KVM re-enable (e.g. after suspend), update clocks"

This reverts commit 677a73a9. This patch was not meant to be merged and
has issues. Revert it.
Requested-by: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kvm/x86.c

@@ -123,6 +123,8 @@ module_param(tsc_tolerance_ppm, uint, S_IRUGO | S_IWUSR);
 unsigned int __read_mostly lapic_timer_advance_ns = 0;
 module_param(lapic_timer_advance_ns, uint, S_IRUGO | S_IWUSR);
 
+static bool __read_mostly backwards_tsc_observed = false;
+
 #define KVM_NR_SHARED_MSRS 16
 
 struct kvm_shared_msrs_global {
@@ -1669,6 +1671,7 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
 					&ka->master_cycle_now);
 
 	ka->use_master_clock = host_tsc_clocksource && vcpus_matched
+				&& !backwards_tsc_observed
 				&& !ka->boot_vcpu_runs_old_kvmclock;
 
 	if (ka->use_master_clock)
@@ -7363,22 +7366,88 @@ int kvm_arch_hardware_enable(void)
 	struct kvm_vcpu *vcpu;
 	int i;
 	int ret;
+	u64 local_tsc;
+	u64 max_tsc = 0;
+	bool stable, backwards_tsc = false;
 
 	kvm_shared_msr_cpu_online();
 	ret = kvm_x86_ops->hardware_enable();
 	if (ret != 0)
 		return ret;
 
+	local_tsc = rdtsc();
+	stable = !check_tsc_unstable();
 	list_for_each_entry(kvm, &vm_list, vm_list) {
 		kvm_for_each_vcpu(i, vcpu, kvm) {
-			if (vcpu->cpu == smp_processor_id()) {
+			if (!stable && vcpu->cpu == smp_processor_id())
 				kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
-				kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE,
-						 vcpu);
+			if (stable && vcpu->arch.last_host_tsc > local_tsc) {
+				backwards_tsc = true;
+				if (vcpu->arch.last_host_tsc > max_tsc)
+					max_tsc = vcpu->arch.last_host_tsc;
+			}
 		}
 	}
+
+	/*
+	 * Sometimes, even reliable TSCs go backwards.  This happens on
+	 * platforms that reset TSC during suspend or hibernate actions, but
+	 * maintain synchronization.  We must compensate.  Fortunately, we can
+	 * detect that condition here, which happens early in CPU bringup,
+	 * before any KVM threads can be running.  Unfortunately, we can't
+	 * bring the TSCs fully up to date with real time, as we aren't yet far
+	 * enough into CPU bringup that we know how much real time has actually
+	 * elapsed; our helper function, get_kernel_ns() will be using boot
+	 * variables that haven't been updated yet.
+	 *
+	 * So we simply find the maximum observed TSC above, then record the
+	 * adjustment to TSC in each VCPU.  When the VCPU later gets loaded,
+	 * the adjustment will be applied.  Note that we accumulate
+	 * adjustments, in case multiple suspend cycles happen before some VCPU
+	 * gets a chance to run again.  In the event that no KVM threads get a
+	 * chance to run, we will miss the entire elapsed period, as we'll have
+	 * reset last_host_tsc, so VCPUs will not have the TSC adjusted and may
+	 * loose cycle time.  This isn't too big a deal, since the loss will be
+	 * uniform across all VCPUs (not to mention the scenario is extremely
+	 * unlikely). It is possible that a second hibernate recovery happens
+	 * much faster than a first, causing the observed TSC here to be
+	 * smaller; this would require additional padding adjustment, which is
+	 * why we set last_host_tsc to the local tsc observed here.
+	 *
+	 * N.B. - this code below runs only on platforms with reliable TSC,
+	 * as that is the only way backwards_tsc is set above.  Also note
+	 * that this runs for ALL vcpus, which is not a bug; all VCPUs should
+	 * have the same delta_cyc adjustment applied if backwards_tsc
+	 * is detected.  Note further, this adjustment is only done once,
+	 * as we reset last_host_tsc on all VCPUs to stop this from being
+	 * called multiple times (one for each physical CPU bringup).
+	 *
+	 * Platforms with unreliable TSCs don't have to deal with this, they
+	 * will be compensated by the logic in vcpu_load, which sets the TSC to
+	 * catchup mode.  This will catchup all VCPUs to real time, but cannot
+	 * guarantee that they stay in perfect synchronization.
+	 */
+	if (backwards_tsc) {
+		u64 delta_cyc = max_tsc - local_tsc;
+		backwards_tsc_observed = true;
+		list_for_each_entry(kvm, &vm_list, vm_list) {
+			kvm_for_each_vcpu(i, vcpu, kvm) {
+				vcpu->arch.tsc_offset_adjustment += delta_cyc;
+				vcpu->arch.last_host_tsc = local_tsc;
+				kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
+			}
+
+			/*
+			 * We have to disable TSC offset matching.. if you were
+			 * booting a VM while issuing an S4 host suspend....
+			 * you may have some problem.  Solving this issue is
+			 * left as an exercise to the reader.
+			 */
+			kvm->arch.last_tsc_nsec = 0;
+			kvm->arch.last_tsc_write = 0;
 		}
 
+	}
 	return 0;
 }