From: Chao Gao <redacted>

The CPU STARTING section doesn't allow callbacks to fail. Move KVM's
hotplug callback to ONLINE section so that it can abort onlining a
CPU in
certain cases to avoid potentially breaking VMs running on existing
CPUs.
For example, when KVM fails to enable hardware virtualization on the
hotplugged CPU.

Place KVM's hotplug state before CPUHP_AP_SCHED_WAIT_EMPTY as it
ensures
when offlining a CPU, all user tasks and non-pinned kernel tasks have
left
the CPU, i.e. there cannot be a vCPU task around. So, it is safe for
KVM's
CPU offline callback to disable hardware virtualization at that
point.
Likewise, KVM's online callback can enable hardware virtualization
before
any vCPU task gets a chance to run on hotplugged CPUs.

Rename KVM's CPU hotplug callbacks accordingly.

Suggested-by: Thomas Gleixner <redacted>
Signed-off-by: Chao Gao <redacted>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Isaku Yamahata <redacted>
Reviewed-by: Yuan Yao <redacted>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 include/linux/cpuhotplug.h |  2 +-
 virt/kvm/kvm_main.c        | 30 ++++++++++++++++++++++--------
 2 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index 7337414e4947..de45be38dd27 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h

@@ -185,7 +185,6 @@ enum cpuhp_state {
 	CPUHP_AP_CSKY_TIMER_STARTING,
 	CPUHP_AP_TI_GP_TIMER_STARTING,
 	CPUHP_AP_HYPERV_TIMER_STARTING,
-	CPUHP_AP_KVM_STARTING,
 	/* Must be the last timer callback */
 	CPUHP_AP_DUMMY_TIMER_STARTING,
 	CPUHP_AP_ARM_XEN_STARTING,

@@ -200,6 +199,7 @@ enum cpuhp_state {
 
 	/* Online section invoked on the hotplugged CPU from the

hotplug thread */
 	CPUHP_AP_ONLINE_IDLE,
+	CPUHP_AP_KVM_ONLINE,
 	CPUHP_AP_SCHED_WAIT_EMPTY,
 	CPUHP_AP_SMPBOOT_THREADS,
 	CPUHP_AP_X86_VDSO_VMA_ONLINE,

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index dd13af9f06d5..fd9e39c85549 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c

@@ -5026,13 +5026,27 @@ static void hardware_enable_nolock(void

*junk)
 	}
 }
 
-static int kvm_starting_cpu(unsigned int cpu)
+static int kvm_online_cpu(unsigned int cpu)
 {
+	int ret = 0;
+
 	raw_spin_lock(&kvm_count_lock);
-	if (kvm_usage_count)
+	/*
+	 * Abort the CPU online process if hardware virtualization
cannot
+	 * be enabled. Otherwise running VMs would encounter
unrecoverable
+	 * errors when scheduled to this CPU.
+	 */
+	if (kvm_usage_count) {
+		WARN_ON_ONCE(atomic_read(&hardware_enable_failed));
+
 		hardware_enable_nolock(NULL);
+		if (atomic_read(&hardware_enable_failed)) {
+			atomic_set(&hardware_enable_failed, 0);

+			ret = -EIO;
+		}
+	}
 	raw_spin_unlock(&kvm_count_lock);
-	return 0;
+	return ret;
 }
 
 static void hardware_disable_nolock(void *junk)

@@ -5045,7 +5059,7 @@ static void hardware_disable_nolock(void *junk)
 	kvm_arch_hardware_disable();
 }
 
-static int kvm_dying_cpu(unsigned int cpu)
+static int kvm_offline_cpu(unsigned int cpu)
 {
 	raw_spin_lock(&kvm_count_lock);
 	if (kvm_usage_count)

@@ -5822,8 +5836,8 @@ int kvm_init(unsigned vcpu_size, unsigned

vcpu_align, struct module *module)
 	if (!zalloc_cpumask_var(&cpus_hardware_enabled, GFP_KERNEL))
 		return -ENOMEM;
 
-	r = cpuhp_setup_state_nocalls(CPUHP_AP_KVM_STARTING,
"kvm/cpu:starting",
-				      kvm_starting_cpu, kvm_dying_cpu);
+	r = cpuhp_setup_state_nocalls(CPUHP_AP_KVM_ONLINE,
"kvm/cpu:online",
+				      kvm_online_cpu, kvm_offline_cpu);
 	if (r)
 		goto out_free_2;
 	register_reboot_notifier(&kvm_reboot_notifier);

@@ -5897,7 +5911,7 @@ int kvm_init(unsigned vcpu_size, unsigned

vcpu_align, struct module *module)
 	kmem_cache_destroy(kvm_vcpu_cache);
 out_free_3:
 	unregister_reboot_notifier(&kvm_reboot_notifier);
-	cpuhp_remove_state_nocalls(CPUHP_AP_KVM_STARTING);
+	cpuhp_remove_state_nocalls(CPUHP_AP_KVM_ONLINE);
 out_free_2:
 	free_cpumask_var(cpus_hardware_enabled);
 	return r;

@@ -5923,7 +5937,7 @@ void kvm_exit(void)
 	kvm_async_pf_deinit();
 	unregister_syscore_ops(&kvm_syscore_ops);
 	unregister_reboot_notifier(&kvm_reboot_notifier);
-	cpuhp_remove_state_nocalls(CPUHP_AP_KVM_STARTING);
+	cpuhp_remove_state_nocalls(CPUHP_AP_KVM_ONLINE);
 	on_each_cpu(hardware_disable_nolock, NULL, 1);
 	kvm_irqfd_exit();
 	free_cpumask_var(cpus_hardware_enabled);