kvm tools: Allow the user to pass a FD to use as a TAP device

This allows users to pass a pre-configured fd to use for the network
interface.

For example:
        kvm run -n mode=tap,fd=3 3<>/dev/net/tap3

Acked-by: Daniel P. Berrange <berrange@redhat.com>
Cc: Osier Yang <jyang@redhat.com>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

builtin-run.c

@@ -220,6 +220,8 @@ static int set_net_param(struct virtio_net_params *p, const char *param,
 		p->host_ip = strdup(val);
 	} else if (strcmp(param, "vhost") == 0) {
 		p->vhost = atoi(val);
+	} else if (strcmp(param, "fd") == 0) {
+		p->fd = atoi(val);
 	}
 
 	return 0;

include/kvm/virtio-net.h

@@ -12,6 +12,7 @@ struct virtio_net_params {
 	struct kvm *kvm;
 	int mode;
 	int vhost;
+	int fd;
 };
 
 void virtio_net__init(const struct virtio_net_params *params);

virtio/net.c

@@ -178,6 +178,12 @@ static bool virtio_net__tap_init(const struct virtio_net_params *params,
 	struct sockaddr_in sin = {0};
 	struct ifreq ifr;
 
+	/* Did the user already gave us the FD? */
+	if (params->fd) {
+		ndev->tap_fd = params->fd;
+		return 1;
+	}
+
 	ndev->tap_fd = open("/dev/net/tun", O_RDWR);
 	if (ndev->tap_fd < 0) {
 		pr_warning("Unable to open /dev/net/tun");