[PATCH v1 1/1] KVM: trigger uevents when starting or stopping a VM

[Claudio Imbrenda <imbrenda@xxxxxxxxxxxxxxxxxx>]

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 two environment variables:
KVM_VM_CREATED which indicates how many times a new VM has been created,
KVM_VM_COUNT which indicates how many VMs are currently active.

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@xxxxxxxxxxxxxxxxxx>
---
 virt/kvm/kvm_main.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 6e3b12c..f67aa59 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -130,6 +130,10 @@ EXPORT_SYMBOL_GPL(kvm_rebooting);
 
 static bool largepages_enabled = true;
 
+static void kvm_uevent_notify_change(u64 created, u64 active);
+static u64 kvm_createvm_count;
+static u64 kvm_active_vms;
+
 bool kvm_is_reserved_pfn(kvm_pfn_t pfn)
 {
 	if (pfn_valid(pfn))
@@ -627,6 +631,7 @@ static struct kvm *kvm_create_vm(unsigned long type)
 {
 	int r, i;
 	struct kvm *kvm = kvm_arch_alloc_vm();
+	u64 activevms, createdvms;
 
 	if (!kvm)
 		return ERR_PTR(-ENOMEM);
@@ -686,9 +691,12 @@ static struct kvm *kvm_create_vm(unsigned long type)
 
 	spin_lock(&kvm_lock);
 	list_add(&kvm->vm_list, &vm_list);
+	createdvms = ++kvm_createvm_count;
+	activevms = ++kvm_active_vms;
 	spin_unlock(&kvm_lock);
 
 	preempt_notifier_inc();
+	kvm_uevent_notify_change(createdvms, activevms);
 
 	return kvm;
 
@@ -739,11 +747,14 @@ static void kvm_destroy_vm(struct kvm *kvm)
 {
 	int i;
 	struct mm_struct *mm = kvm->mm;
+	u64 activevms, createdvms;
 
 	kvm_destroy_vm_debugfs(kvm);
 	kvm_arch_sync_events(kvm);
 	spin_lock(&kvm_lock);
 	list_del(&kvm->vm_list);
+	activevms = --kvm_active_vms;
+	createdvms = kvm_createvm_count;
 	spin_unlock(&kvm_lock);
 	kvm_free_irq_routing(kvm);
 	for (i = 0; i < KVM_NR_BUSES; i++) {
@@ -767,6 +778,7 @@ static void kvm_destroy_vm(struct kvm *kvm)
 	preempt_notifier_dec();
 	hardware_disable_all();
 	mmdrop(mm);
+	kvm_uevent_notify_change(createdvms, activevms);
 }
 
 void kvm_get_kvm(struct kvm *kvm)
@@ -3864,6 +3876,19 @@ static const struct file_operations *stat_fops[] = {
 	[KVM_STAT_VM]   = &vm_stat_fops,
 };
 
+static void kvm_uevent_notify_change(u64 created, u64 active)
+{
+	char createvm_buf[40];
+	char activevm_buf[40];
+	char *ptr[3] = {createvm_buf, activevm_buf, NULL};
+
+	if (!kvm_dev.this_device)
+		return;
+	snprintf(createvm_buf, 40, "KVM_VM_CREATED=%llu", created);
+	snprintf(activevm_buf, 40, "KVM_VM_ACTIVE=%llu", active);
+	kobject_uevent_env(&kvm_dev.this_device->kobj, KOBJ_CHANGE, ptr);
+}
+
 static int kvm_init_debug(void)
 {
 	int r = -EEXIST;
-- 
2.7.4