KVM: trigger uevents when creating or destroying a VM

This patch adds a few lines to the KVM common code to fire a
KOBJ_CHANGE uevent whenever a KVM VM is created or destroyed. The event
carries five environment variables:

CREATED indicates how many times a new VM has been created. It is
	useful for example to trigger specific actions when the first
	VM is started
COUNT indicates how many VMs are currently active. This can be used for
	logging or monitoring purposes
PID has the pid of the KVM process that has been started or stopped.
	This can be used to perform process-specific tuning.
STATS_PATH contains the path in debugfs to the directory with all the
	runtime statistics for this VM. This is useful for performance
	monitoring and profiling.
EVENT described the type of event, its value can be either "create" or
	"destroy"

Specific udev rules can be then set up in userspace to deal with the
creation or destruction of VMs as needed.
Signed-off-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

virt/kvm/kvm_main.c

@@ -130,6 +130,12 @@ EXPORT_SYMBOL_GPL(kvm_rebooting);
 
 static bool largepages_enabled = true;
 
+#define KVM_EVENT_CREATE_VM 0
+#define KVM_EVENT_DESTROY_VM 1
+static void kvm_uevent_notify_change(unsigned int type, struct kvm *kvm);
+static unsigned long long kvm_createvm_count;
+static unsigned long long kvm_active_vms;
+
 bool kvm_is_reserved_pfn(kvm_pfn_t pfn)
 {
 	if (pfn_valid(pfn))
@@ -740,6 +746,7 @@ static void kvm_destroy_vm(struct kvm *kvm)
 	int i;
 	struct mm_struct *mm = kvm->mm;
 
+	kvm_uevent_notify_change(KVM_EVENT_DESTROY_VM, kvm);
 	kvm_destroy_vm_debugfs(kvm);
 	kvm_arch_sync_events(kvm);
 	spin_lock(&kvm_lock);
@@ -3220,6 +3227,7 @@ static int kvm_dev_ioctl_create_vm(unsigned long type)
 		fput(file);
 		return -ENOMEM;
 	}
+	kvm_uevent_notify_change(KVM_EVENT_CREATE_VM, kvm);
 
 	fd_install(r, file);
 	return r;
@@ -3872,6 +3880,67 @@ static const struct file_operations *stat_fops[] = {
 	[KVM_STAT_VM]   = &vm_stat_fops,
 };
 
+static void kvm_uevent_notify_change(unsigned int type, struct kvm *kvm)
+{
+	struct kobj_uevent_env *env;
+	char *tmp, *pathbuf = NULL;
+	unsigned long long created, active;
+
+	if (!kvm_dev.this_device || !kvm)
+		return;
+
+	spin_lock(&kvm_lock);
+	if (type == KVM_EVENT_CREATE_VM) {
+		kvm_createvm_count++;
+		kvm_active_vms++;
+	} else if (type == KVM_EVENT_DESTROY_VM) {
+		kvm_active_vms--;
+	}
+	created = kvm_createvm_count;
+	active = kvm_active_vms;
+	spin_unlock(&kvm_lock);
+
+	env = kzalloc(sizeof(*env), GFP_KERNEL);
+	if (!env)
+		return;
+
+	add_uevent_var(env, "CREATED=%llu", created);
+	add_uevent_var(env, "COUNT=%llu", active);
+
+	if (type == KVM_EVENT_CREATE_VM)
+		add_uevent_var(env, "EVENT=create");
+	else if (type == KVM_EVENT_DESTROY_VM)
+		add_uevent_var(env, "EVENT=destroy");
+
+	if (kvm->debugfs_dentry) {
+		char p[ITOA_MAX_LEN];
+
+		snprintf(p, sizeof(p), "%s", kvm->debugfs_dentry->d_name.name);
+		tmp = strchrnul(p + 1, '-');
+		*tmp = '\0';
+		add_uevent_var(env, "PID=%s", p);
+		pathbuf = kmalloc(PATH_MAX, GFP_KERNEL);
+		if (pathbuf) {
+			/* sizeof counts the final '\0' */
+			int len = sizeof("STATS_PATH=") - 1;
+			const char *pvar = "STATS_PATH=";
+
+			tmp = dentry_path_raw(kvm->debugfs_dentry,
+					      pathbuf + len,
+					      PATH_MAX - len);
+			if (!IS_ERR(tmp)) {
+				memcpy(tmp - len, pvar, len);
+				env->envp[env->envp_idx++] = tmp - len;
+			}
+		}
+	}
+	/* no need for checks, since we are adding at most only 5 keys */
+	env->envp[env->envp_idx++] = NULL;
+	kobject_uevent_env(&kvm_dev.this_device->kobj, KOBJ_CHANGE, env->envp);
+	kfree(env);
+	kfree(pathbuf);
+}
+
 static int kvm_init_debug(void)
 {
 	int r = -EEXIST;