xen: introduce xen_console_io option

Xen can support console_io hypercalls for any domains, not just dom0,
depending on DEBUG and XSM policies. These hypercalls can be very useful
for development and debugging.

Introduce a kernel command line option xen_console_io to enable the
usage of console_io hypercalls for any domain upon request. When
xen_console_io is not specified, the current behavior is retained.

Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <alpine.DEB.2.22.394.2601131522540.992863@ubuntu-linux-20-04-desktop>

Author: sstabellini

Documentation/admin-guide/kernel-parameters.txt

@@ -8414,6 +8414,11 @@ Kernel parameters
 			save/restore/migration must be enabled to handle larger
 			domains.
 
+	xen_console_io	[XEN,EARLY]
+			Boolean option to enable/disable the usage of the Xen
+			console_io hypercalls to read and write to the console.
+			Mostly useful for debugging and development.
+
 	xen_emul_unplug=		[HW,X86,XEN,EARLY]
 			Unplug Xen emulated devices
 			Format: [unplug0,][unplug1]

drivers/tty/hvc/hvc_xen.c

@@ -51,6 +51,22 @@ static DEFINE_SPINLOCK(xencons_lock);
 
 /* ------------------------------------------------------------------ */
 
+static bool xen_console_io = false;
+static int __initdata opt_console_io = -1;
+
+static int __init parse_xen_console_io(char *arg)
+{
+	bool val;
+	int ret;
+
+	ret = kstrtobool(arg, &val);
+	if (ret == 0)
+		opt_console_io = (int)val;
+
+	return ret;
+}
+early_param("xen_console_io", parse_xen_console_io);
+
 static struct xencons_info *vtermno_to_xencons(int vtermno)
 {
 	struct xencons_info *entry, *ret = NULL;
@@ -331,7 +347,7 @@ static int xen_initial_domain_console_init(void)
 	struct xencons_info *info;
 	unsigned long flags;
 
-	if (!xen_initial_domain())
+	if (!xen_console_io)
 		return -ENODEV;
 
 	info = vtermno_to_xencons(HVC_COOKIE);
@@ -369,7 +385,7 @@ void xen_console_resume(void)
 {
 	struct xencons_info *info = vtermno_to_xencons(HVC_COOKIE);
 	if (info != NULL && info->irq) {
-		if (!xen_initial_domain())
+		if (!xen_console_io)
 			xen_console_update_evtchn(info);
 		rebind_evtchn_irq(info->evtchn, info->irq);
 	}
@@ -601,7 +617,7 @@ static int __init xen_hvc_init(void)
 	if (!xen_domain())
 		return -ENODEV;
 
-	if (xen_initial_domain()) {
+	if (xen_console_io) {
 		ops = &dom0_hvc_ops;
 		r = xen_initial_domain_console_init();
 		if (r < 0)
@@ -647,14 +663,17 @@ static int __init xen_hvc_init(void)
 }
 device_initcall(xen_hvc_init);
 
-static int xen_cons_init(void)
+static int __init xen_cons_init(void)
 {
 	const struct hv_ops *ops;
 
+	xen_console_io = opt_console_io >= 0 ? opt_console_io :
+					       xen_initial_domain();
+
 	if (!xen_domain())
 		return 0;
 
-	if (xen_initial_domain())
+	if (xen_console_io)
 		ops = &dom0_hvc_ops;
 	else {
 		int r;