KVM: x86: Fold guts of kvm_arch_sync_events() into kvm_arch_pre_destroy_vm()

Fold the guts of kvm_arch_sync_events() into kvm_arch_pre_destroy_vm(), as
the kvmclock and PIT background workers only need to be stopped before
destroying vCPUs (to avoid accessing vCPUs as they are being freed); it's
a-ok for them to be running while the VM is visible on the global vm_list.

Note, the PIT also needs to be stopped before IRQ routing is freed
(because KVM's IRQ routing is garbage and assumes there is always non-NULL
routing).

Opportunistically add comments to explain why KVM stops/frees certain
assets early.

Signed-off-by: Sean Christopherson <[e-mail]>
Message-ID: <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

arch/x86/kvm/x86.c

···
 
 void kvm_arch_sync_events(struct kvm *kvm)
 {
-	cancel_delayed_work_sync(&kvm->arch.kvmclock_sync_work);
-	cancel_delayed_work_sync(&kvm->arch.kvmclock_update_work);
-	kvm_free_pit(kvm);
+
 }
 
 /**
···
 
 void kvm_arch_pre_destroy_vm(struct kvm *kvm)
 {
+	/*
+	 * Stop all background workers and kthreads before destroying vCPUs, as
+	 * iterating over vCPUs in a different task while vCPUs are being freed
+	 * is unsafe, i.e. will lead to use-after-free.  The PIT also needs to
+	 * be stopped before IRQ routing is freed.
+	 */
+	cancel_delayed_work_sync(&kvm->arch.kvmclock_sync_work);
+	cancel_delayed_work_sync(&kvm->arch.kvmclock_update_work);
+
+	kvm_free_pit(kvm);
+
 	kvm_mmu_pre_destroy_vm(kvm);
 }