This patch adds a new option to the input-linux object:

grab-toggle=[key-combo]

The key combination can be one of the following:

* ctrl-ctrl
* alt-alt
* meta-meta
* ctrl-alt
* scrolllock
* ctrl-scrolllock
* ctrl-backtick

The user can pick any of these key combinations. The VM's grab
of the evdev device will be toggled when the key combination is
pressed.

Any invalid setting will result in an error. No setting will
result in the current default of ctrl-ctrl.

If scrolllock is selected as one of the grab-toggle keys, it
will be entirely disabled and not passed to the guest at all.
This is to prevent enabling it while attempting to leave or enter
the VM. On the host, scrolllock can be disabled using xmodmap.

First, find the modifier that Scroll_Lock is bound to:

$ xmodmap -pm

Then, remove Scroll_Lock from it, replacing modX with the modifier:

$ xmodmap -e 'remove modX = Scroll_Lock'

If Scroll_Lock is not bound to any modifier, it is already disabled.

To save the changes, add them to your xinitrc.

Side note - I have changed e-mail addresses from
lightn1ngstr1ke@protonmail.com due to issues with git send-mail.
I am the same person.

Signed-off-by: Ryan El Kochta <relkochta@gmail.com>
---
 qapi/ui.json     | 10 +++++++
 ui/input-linux.c | 71 ++++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 78 insertions(+), 3 deletions(-)

diff --git a/qapi/ui.json b/qapi/ui.json
index 5ad1324..487653a 100644
--- a/qapi/ui.json
+++ b/qapi/ui.json
@@ -1016,6 +1016,16 @@
             '*head'  : 'int',
             'events' : [ 'InputEvent' ] } }
 
+##
+# @GrabToggleKeys:
+#
+# Keys to toggle input-linux between host and guest.
+#
+# Since: 4.0
+#
+##
+{ 'enum': 'GrabToggleKeys',
+  'data': [ 'ctrl-ctrl', 'alt-alt', 'meta-meta', 'ctrl-alt', 'scrolllock', 'ctrl-scrolllock', 'ctrl-backtick' ] }
 
 ##
 # @DisplayGTK:
diff --git a/ui/input-linux.c b/ui/input-linux.c
index 9720333..65d1fb9 100644
--- a/ui/input-linux.c
+++ b/ui/input-linux.c
@@ -63,6 +63,8 @@ struct InputLinux {
     struct input_event event;
     int         read_offset;
 
+    enum GrabToggleKeys grab_toggle;
+
     QTAILQ_ENTRY(InputLinux) next;
 };
 
@@ -98,6 +100,51 @@ static void input_linux_toggle_grab(InputLinux *il)
     }
 }
 
+static bool input_linux_check_toggle(InputLinux *il)
+{
+    switch (il->grab_toggle) {
+    case GRAB_TOGGLE_KEYS_CTRL_CTRL:
+        return il->keydown[KEY_LEFTCTRL] &&
+            il->keydown[KEY_RIGHTCTRL];
+
+    case GRAB_TOGGLE_KEYS_ALT_ALT:
+        return il->keydown[KEY_LEFTALT] &&
+            il->keydown[KEY_RIGHTALT];
+
+    case GRAB_TOGGLE_KEYS_META_META:
+        return il->keydown[KEY_LEFTMETA] &&
+            il->keydown[KEY_RIGHTMETA];
+
+    case GRAB_TOGGLE_KEYS_CTRL_ALT:
+        return il->keydown[KEY_LEFTCTRL] &&
+            il->keydown[KEY_LEFTALT];
+
+    case GRAB_TOGGLE_KEYS_SCROLLLOCK:
+        return il->keydown[KEY_SCROLLLOCK];
+
+    case GRAB_TOGGLE_KEYS_CTRL_SCROLLLOCK:
+        return il->keydown[KEY_LEFTCTRL] &&
+            il->keydown[KEY_SCROLLLOCK];
+
+    case GRAB_TOGGLE_KEYS_CTRL_BACKTICK:
+        return il->keydown[KEY_LEFTCTRL] &&
+            il->keydown[KEY_GRAVE];
+
+    case GRAB_TOGGLE_KEYS__MAX:
+        /* avoid gcc error */
+        break;
+    }
+    return false;
+}
+
+static bool input_linux_should_skip(InputLinux *il,
+                                    struct input_event *event)
+{
+    return (il->grab_toggle == GRAB_TOGGLE_KEYS_SCROLLLOCK ||
+            il->grab_toggle == GRAB_TOGGLE_KEYS_CTRL_SCROLLLOCK) &&
+            event->code == KEY_SCROLLLOCK;
+}
+
 static void input_linux_handle_keyboard(InputLinux *il,
                                         struct input_event *event)
 {
@@ -128,14 +175,13 @@ static void input_linux_handle_keyboard(InputLinux *il,
         }
 
         /* send event to guest when grab is active */
-        if (il->grab_active) {
+        if (il->grab_active && !input_linux_should_skip(il, event)) {
             int qcode = qemu_input_linux_to_qcode(event->code);
             qemu_input_event_send_key_qcode(NULL, qcode, event->value);
         }
 
         /* hotkey -> record switch request ... */
-        if (il->keydown[KEY_LEFTCTRL] &&
-            il->keydown[KEY_RIGHTCTRL]) {
+        if (input_linux_check_toggle(il)) {
             il->grab_request = true;
         }
 
@@ -410,6 +456,21 @@ static void input_linux_set_repeat(Object *obj, bool value,
     il->repeat = value;
 }
 
+static int input_linux_get_grab_toggle(Object *obj, Error **errp)
+{
+    InputLinux *il = INPUT_LINUX(obj);
+
+    return il->grab_toggle;
+}
+
+static void input_linux_set_grab_toggle(Object *obj, int value,
+                                       Error **errp)
+{
+    InputLinux *il = INPUT_LINUX(obj);
+
+    il->grab_toggle = value;
+}
+
 static void input_linux_instance_init(Object *obj)
 {
     object_property_add_str(obj, "evdev",
@@ -421,6 +482,10 @@ static void input_linux_instance_init(Object *obj)
     object_property_add_bool(obj, "repeat",
                              input_linux_get_repeat,
                              input_linux_set_repeat, NULL);
+    object_property_add_enum(obj, "grab-toggle", "GrabToggleKeys",
+                             &GrabToggleKeys_lookup,
+                             input_linux_get_grab_toggle,
+                             input_linux_set_grab_toggle, NULL);
 }
 
 static void input_linux_class_init(ObjectClass *oc, void *data)
-- 
2.20.1

On Sat, Jan 19, 2019 at 11:14:27PM -0500, Ryan El Kochta wrote:
> This patch adds a new option to the input-linux object:
> 
> grab-toggle=[key-combo]
> 
> The key combination can be one of the following:
> 
> * ctrl-ctrl

left ctrl + right ctrl.

> * alt-alt

Same.

> * meta-meta

Same.

> * ctrl-alt

This is actually left ctrl + left alt (not any ctrl / alt) in the code.

Ctrl+Alt+Key isn't that uncommon as hotkey, linux console switching uses
Ctrl+Alt+Fn for example.  Also some GUI apps use Ctrl+Alt+Someletter for
less frequently used functions when they run out of Ctrl+Someletter
hotkeys.  I'd suggest to drop this one.

> * scrolllock
> * ctrl-scrolllock

This likewise is actually left ctrl + scrolllock in the code.

> * ctrl-backtick

Not a good idea.  backtick has no fixed location, depends on the
keyboard layout.

So, the naming is a bit tricky here.  If only the left ctrl key actually
works we should say so in the name and call it "lctrl-scrolllock".  Or
leave the name and allow both ctrl keys.

For the ctrl-ctrl (and simliar) hotkeys I think it doesn't matter much
whenever we call them "ctrl-ctrl" or "lctrl-rctrl".  It is kida obvious
that you need both the left and right one when you should press two
control keys.  But I think it should be consistent with what we pick for
ctrl-scrolllock.

I think sticking with the current names and make both ctrl keys work
with scrolllock is the better way.

cheers,
  Gerd