[PATCH v2 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 four environment variables:

KVM_VM_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
KVM_VM_COUNT indicates how many VMs are currently active. This can be
             used for logging or monitoring purposes
KVM_VM_PID has the pid of the KVM process that has been started or
           stopped. This can be used to perform process-specific
           tuning.
KVM_VM_STATS_PATH contains the path in /sys to the directory with all
                  the runtime statistics for this VM. This is useful
                  for performance monitoring and profiling.

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 | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 6e3b12c..f4cf7d2 100644
--- a/virt/kvm/kvm_main.c
+++ b/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);
@@ -3212,6 +3219,7 @@ 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;
@@ -3864,6 +3872,48 @@ 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)
+{
+	char cbuf[40], abuf[40], pidbuf[40], pathvar[18] = "KVM_VM_STATS_PATH=";
+	char *ptr[5] = {cbuf, abuf, pidbuf, NULL, NULL};
+	char *tmp, *pathbuf;
+	unsigned long long created, active;
+
+	if (!kvm_dev.this_device || !kvm || !kvm->debugfs_dentry)
+		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);
+
+	pathbuf = kmalloc(PATH_MAX, GFP_KERNEL);
+	if (pathbuf) {
+		tmp = dentry_path_raw(kvm->debugfs_dentry,
+				      pathbuf + sizeof(pathvar),
+				      PATH_MAX - sizeof(pathvar));
+		if (!IS_ERR(tmp)) {
+			memcpy(tmp - sizeof(pathvar), pathvar, sizeof(pathvar));
+			ptr[3] = tmp - sizeof(pathvar);
+		}
+	}
+	snprintf(cbuf, sizeof(cbuf), "KVM_VM_CREATED=%llu", created);
+	snprintf(abuf, sizeof(abuf), "KVM_VM_ACTIVE=%llu", active);
+	snprintf(pidbuf, sizeof(pidbuf), "KVM_VM_PID=%s",
+		 kvm->debugfs_dentry->d_name.name);
+	tmp = strchr(pidbuf, '-');
+	if (tmp)
+		*tmp = '\0';
+	kobject_uevent_env(&kvm_dev.this_device->kobj, KOBJ_CHANGE, ptr);
+	kfree(pathbuf);
+}
+
 static int kvm_init_debug(void)
 {
 	int r = -EEXIST;
-- 
2.7.4