um: virtio_uml: Support adding devices via mconsole

It can be used when we want to add virtio devices to UML while
it's up and running. Virtio devices can be added to UML using
the same syntax as the command line option:

(mconsole) config virtio_uml.device=<socket>:<virtio_id>[:<platform_id>]

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20260106004259.1604736-1-tiwei.btw@antgroup.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Author: btw616

arch/um/drivers/virtio_uml.c

@@ -34,6 +34,7 @@
 #include <irq_kern.h>
 #include <init.h>
 #include <os.h>
+#include "mconsole_kern.h"
 #include "vhost_user.h"
 
 #define MAX_SUPPORTED_QUEUE_SIZE	256
@@ -1282,6 +1283,7 @@ static struct device vu_cmdline_parent = {
 	.release = vu_cmdline_release_dev,
 };
 
+static DEFINE_MUTEX(vu_cmdline_lock);
 static bool vu_cmdline_parent_registered;
 static int vu_cmdline_id;
 
@@ -1309,7 +1311,7 @@ static void vu_conn_broken(struct work_struct *wk)
 	vu_unregister_cmdline_device(&pdata->pdev->dev, NULL);
 }
 
-static int vu_cmdline_set(const char *device, const struct kernel_param *kp)
+static int vu_cmdline_set_device(const char *device)
 {
 	const char *ids = strchr(device, ':');
 	unsigned int virtio_device_id;
@@ -1321,6 +1323,8 @@ static int vu_cmdline_set(const char *device, const struct kernel_param *kp)
 	if (!ids || ids == device)
 		return -EINVAL;
 
+	guard(mutex)(&vu_cmdline_lock);
+
 	processed = sscanf(ids, ":%u%n:%d%n",
 			   &virtio_device_id, &consumed,
 			   &vu_cmdline_id, &consumed);
@@ -1366,6 +1370,11 @@ static int vu_cmdline_set(const char *device, const struct kernel_param *kp)
 	return err;
 }
 
+static int vu_cmdline_set(const char *device, const struct kernel_param *kp)
+{
+	return vu_cmdline_set_device(device);
+}
+
 static int vu_cmdline_get_device(struct device *dev, void *data)
 {
 	struct platform_device *pdev = to_platform_device(dev);
@@ -1380,6 +1389,8 @@ static int vu_cmdline_get_device(struct device *dev, void *data)
 
 static int vu_cmdline_get(char *buffer, const struct kernel_param *kp)
 {
+	guard(mutex)(&vu_cmdline_lock);
+
 	buffer[0] = '\0';
 	if (vu_cmdline_parent_registered)
 		device_for_each_child(&vu_cmdline_parent, buffer,
@@ -1403,6 +1414,8 @@ __uml_help(vu_cmdline_param_ops,
 
 static void vu_unregister_cmdline_devices(void)
 {
+	guard(mutex)(&vu_cmdline_lock);
+
 	if (vu_cmdline_parent_registered) {
 		device_for_each_child(&vu_cmdline_parent, NULL,
 				      vu_unregister_cmdline_device);
@@ -1411,6 +1424,42 @@ static void vu_unregister_cmdline_devices(void)
 	}
 }
 
+static int vu_mc_config(char *str, char **error_out)
+{
+	if (*str != '=') {
+		*error_out = "Invalid config";
+		return -EINVAL;
+	}
+	str += 1;
+	return vu_cmdline_set_device(str);
+}
+
+static int vu_mc_id(char **str, int *start_out, int *end_out)
+{
+	return -EOPNOTSUPP;
+}
+
+static int vu_mc_remove(int n, char **error_out)
+{
+	return -EOPNOTSUPP;
+}
+
+static struct mc_device virtio_uml_mc = {
+	.list		= LIST_HEAD_INIT(virtio_uml_mc.list),
+	.name		= "virtio_uml.device",
+	.config		= vu_mc_config,
+	.get_config	= NULL,
+	.id		= vu_mc_id,
+	.remove		= vu_mc_remove,
+};
+
+static int __init virtio_uml_mc_init(void)
+{
+	mconsole_register_dev(&virtio_uml_mc);
+	return 0;
+}
+late_initcall(virtio_uml_mc_init);
+
 /* Platform driver */
 
 static const struct of_device_id virtio_uml_match[] = {