[PATCH v3 27/41] x86/kvmclock: Enable kvmclock on APs during onlining if kvmclock isn't sched_clock

[Sean Christopherson <seanjc@google.com>]

In anticipation of making x86_cpuinit.early_percpu_clock_init(), i.e.
kvm_setup_secondary_clock(), a dedicated sched_clock hook that will be
invoked if and only if kvmclock is set as sched_clock, ensure APs enable
their kvmclock during CPU online.  While a redundant write to the MSR is
technically ok, skip the registration when kvmclock is sched_clock so that
it's somewhat obvious that kvmclock *needs* to be enabled during early
bringup when it's being used as sched_clock.

Plumb in the BSP's resume path purely for documentation purposes.  Both
KVM (as-a-guest) and timekeeping/clocksource hook syscore_ops, and it's
not super obvious that using KVM's hooks would be flawed.  E.g. it would
work today, because KVM's hooks happen to run after/before timekeeping's
hooks during suspend/resume, but that's sheer dumb luck as the order in
which syscore_ops are invoked depends entirely on when a subsystem is
initialized and thus registers its hooks.

Opportunsitically make the registration messages more precise to help
debug issues where kvmclock is enabled too late.

Opportunstically WARN in kvmclock_{suspend,resume}() to harden against
future bugs.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/include/asm/kvm_para.h |  2 ++
 arch/x86/kernel/kvm.c           | 24 +++++++++++-------
 arch/x86/kernel/kvmclock.c      | 44 +++++++++++++++++++++++++++------
 3 files changed, 53 insertions(+), 17 deletions(-)

diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
index 2adba2aff539..17053d2bf270 100644
--- a/arch/x86/include/asm/kvm_para.h
+++ b/arch/x86/include/asm/kvm_para.h
@@ -120,6 +120,8 @@ static inline long kvm_sev_hypercall3(unsigned int nr, unsigned long p1,
 #ifdef CONFIG_KVM_GUEST
 enum kvm_guest_cpu_action {
 	KVM_GUEST_BSP_SUSPEND,
+	KVM_GUEST_BSP_RESUME,
+	KVM_GUEST_AP_ONLINE,
 	KVM_GUEST_AP_OFFLINE,
 	KVM_GUEST_SHUTDOWN,
 };
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 0131bc1cb459..65c787b1ea03 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -471,18 +471,24 @@ static void kvm_guest_cpu_offline(enum kvm_guest_cpu_action action)
 	kvmclock_cpu_action(action);
 }
 
+static int __kvm_cpu_online(unsigned int cpu, enum kvm_guest_cpu_action action)
+{
+	unsigned long flags;
+
+	local_irq_save(flags);
+	kvmclock_cpu_action(action);
+	kvm_guest_cpu_init();
+	local_irq_restore(flags);
+	return 0;
+}
+
+#ifdef CONFIG_SMP
+
 static int kvm_cpu_online(unsigned int cpu)
 {
-	unsigned long flags;
-
-	local_irq_save(flags);
-	kvm_guest_cpu_init();
-	local_irq_restore(flags);
-	return 0;
+	return __kvm_cpu_online(cpu, KVM_GUEST_AP_ONLINE);
 }
 
-#ifdef CONFIG_SMP
-
 static DEFINE_PER_CPU(cpumask_var_t, __pv_cpu_mask);
 
 static bool pv_tlb_flush_supported(void)
@@ -747,7 +753,7 @@ static int kvm_suspend(void *data)
 
 static void kvm_resume(void *data)
 {
-	kvm_cpu_online(raw_smp_processor_id());
+	__kvm_cpu_online(raw_smp_processor_id(), KVM_GUEST_BSP_RESUME);
 
 #ifdef CONFIG_ARCH_CPUIDLE_HALTPOLL
 	if (kvm_para_has_feature(KVM_FEATURE_POLL_CONTROL) && has_guest_poll)
diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index e7250d21c672..d3bb281c0805 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -53,6 +53,7 @@ static struct pvclock_vsyscall_time_info *hvclock_mem;
 DEFINE_PER_CPU(struct pvclock_vsyscall_time_info *, hv_clock_per_cpu);
 EXPORT_PER_CPU_SYMBOL_GPL(hv_clock_per_cpu);
 
+static bool kvmclock_is_sched_clock;
 static bool kvmclock_suspended;
 
 /*
@@ -129,7 +130,7 @@ static void kvm_save_sched_clock_state(void)
 #ifdef CONFIG_SMP
 static void kvm_setup_secondary_clock(void)
 {
-	kvm_register_clock("secondary cpu clock");
+	kvm_register_clock("secondary cpu, sched_clock setup");
 }
 #endif
 
@@ -141,25 +142,51 @@ static void kvm_restore_sched_clock_state(void)
 
 static void kvmclock_suspend(struct clocksource *cs)
 {
+	if (WARN_ON_ONCE(kvmclock_is_sched_clock))
+		return;
+
 	kvmclock_suspended = true;
 	kvmclock_disable();
 }
 
 static void kvmclock_resume(struct clocksource *cs)
 {
+	if (WARN_ON_ONCE(kvmclock_is_sched_clock))
+		return;
+
 	kvmclock_suspended = false;
 	kvm_register_clock("primary cpu, clocksource resume");
 }
 
 void kvmclock_cpu_action(enum kvm_guest_cpu_action action)
 {
-	/*
-	 * Don't disable kvmclock on the BSP during suspend.  If kvmclock is
-	 * being used for sched_clock, then it needs to be kept alive until the
-	 * last minute, and restored as quickly as possible after resume.
-	 */
-	if (action != KVM_GUEST_BSP_SUSPEND)
+	switch (action) {
+		/*
+		 * The BSP's clock is managed via clocksource suspend/resume,
+		 * to ensure it's enabled/disabled when timekeeping needs it
+		 * to be, e.g. before reading wallclock (which uses kvmclock).
+		 */
+	case KVM_GUEST_BSP_SUSPEND:
+	case KVM_GUEST_BSP_RESUME:
+		break;
+	case KVM_GUEST_AP_ONLINE:
+		/*
+		 * Secondary CPUs use dedicated sched_clock hooks to enable
+		 * kvmclock early during bringup, there's nothing to be done
+		 * when during CPU online.
+		 */
+		if (kvmclock_is_sched_clock)
+			break;
+		kvm_register_clock("secondary cpu, online");
+		break;
+	case KVM_GUEST_AP_OFFLINE:
+	case KVM_GUEST_SHUTDOWN:
 		kvmclock_disable();
+		break;
+	default:
+		WARN_ON_ONCE(1);
+		break;
+	}
 }
 
 /*
@@ -315,6 +342,7 @@ static __init void kvm_sched_clock_init(bool stable)
 	__paravirt_set_sched_clock(kvm_sched_clock_read, stable,
 				   kvm_save_sched_clock_state,
 				   kvm_restore_sched_clock_state);
+	kvmclock_is_sched_clock = true;
 
 	/*
 	 * The BSP's clock is managed via dedicated sched_clock save/restore
@@ -358,7 +386,7 @@ void __init kvmclock_init(void)
 		msr_kvm_system_time, msr_kvm_wall_clock);
 
 	this_cpu_write(hv_clock_per_cpu, &hv_clock_boot[0]);
-	kvm_register_clock("primary cpu clock");
+	kvm_register_clock("primary cpu, online");
 	pvclock_set_pvti_cpu0_va(hv_clock_boot);
 
 	if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT)) {
-- 
2.54.0.563.g4f69b47b94-goog