[PATCH v2 1/2] KVM: x86: Make the kvmclock CPU hotplug state generic

[Jinmeng Zhou <jinmengzhou22@gmail.com>]

From: Jinmeng Zhou <zhoujinmeng@bytedance.com>

Rename CPUHP_AP_X86_KVM_CLK_ONLINE to CPUHP_AP_X86_KVM_ONLINE so that the
state can be used for any x86 KVM work that needs to run when a CPU comes
online or goes offline, not just for kvmclock.

The state is currently registered by kvm_timer_init() only if the host
lacks X86_FEATURE_CONSTANT_TSC, so it is never installed on modern CPUs.
Register it unconditionally from kvm_x86_vendor_init(), and remove it from
kvm_x86_vendor_exit().  Add kvm_x86_cpu_online() and kvm_x86_cpu_offline()
wrappers that call the existing kvmclock helpers only on hosts without a
constant TSC.

Keep the registration after kvm_timer_init() so that the cpufreq notifier
is registered before kvmclock_cpu_online() samples the frequency of each
online CPU.

No functional change for kvmclock.  On hosts with a constant TSC, the
state is now registered but its callbacks do nothing.

Cc: stable@vger.kernel.org
Co-developed-by: Guixiong Wei <weiguixiong@bytedance.com>
Signed-off-by: Guixiong Wei <weiguixiong@bytedance.com>
Signed-off-by: Jinmeng Zhou <zhoujinmeng@bytedance.com>
---
 arch/x86/kvm/x86.c         | 29 +++++++++++++++++++++++++----
 include/linux/cpuhotplug.h |  2 +-
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 79468ddfe4736..a08ae25f373fb 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6812,6 +6812,22 @@ static int kvmclock_cpu_online(unsigned int cpu)
 	return 0;
 }
 
+static int kvm_x86_cpu_online(unsigned int cpu)
+{
+	if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
+		return kvmclock_cpu_online(cpu);
+
+	return 0;
+}
+
+static int kvm_x86_cpu_offline(unsigned int cpu)
+{
+	if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
+		return kvmclock_cpu_down_prep(cpu);
+
+	return 0;
+}
+
 static void kvm_timer_init(void)
 {
 	if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
@@ -6832,9 +6848,6 @@ static void kvm_timer_init(void)
 		}
 		cpufreq_register_notifier(&kvmclock_cpufreq_notifier_block,
 					  CPUFREQ_TRANSITION_NOTIFIER);
-
-		cpuhp_setup_state(CPUHP_AP_X86_KVM_CLK_ONLINE, "x86/kvm/clk:online",
-				  kvmclock_cpu_online, kvmclock_cpu_down_prep);
 	}
 }
 
@@ -7109,6 +7122,13 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
 	 */
 	kvm_timer_init();
 
+	/*
+	 * Register the CPU hotplug state after the cpufreq notifier so that
+	 * kvmclock_cpu_online() can't miss a frequency transition.
+	 */
+	cpuhp_setup_state(CPUHP_AP_X86_KVM_ONLINE, "x86/kvm:online",
+			  kvm_x86_cpu_online, kvm_x86_cpu_offline);
+
 	if (pi_inject_timer == -1)
 		pi_inject_timer = housekeeping_enabled(HK_TYPE_TIMER);
 #ifdef CONFIG_X86_64
@@ -7165,10 +7185,11 @@ void kvm_x86_vendor_exit(void)
 #endif
 	kvm_lapic_exit();
 
+	cpuhp_remove_state_nocalls(CPUHP_AP_X86_KVM_ONLINE);
+
 	if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
 		cpufreq_unregister_notifier(&kvmclock_cpufreq_notifier_block,
 					    CPUFREQ_TRANSITION_NOTIFIER);
-		cpuhp_remove_state_nocalls(CPUHP_AP_X86_KVM_CLK_ONLINE);
 	}
 #ifdef CONFIG_X86_64
 	pvclock_gtod_unregister_notifier(&pvclock_gtod_notifier);
diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index feb32949aeea6..91da42fd11161 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -241,7 +241,7 @@ enum cpuhp_state {
 	CPUHP_AP_ONLINE_DYN,
 	CPUHP_AP_ONLINE_DYN_END		= CPUHP_AP_ONLINE_DYN + 40,
 	CPUHP_AP_X86_HPET_ONLINE,
-	CPUHP_AP_X86_KVM_CLK_ONLINE,
+	CPUHP_AP_X86_KVM_ONLINE,
 	CPUHP_AP_ACTIVE,
 	CPUHP_ONLINE,
 };
-- 
2.39.5