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Re: [Qemu-devel] Request for comment - dynamic VNC keyboard mapping

From: Daniel Henrique Barboza
Subject: Re: [Qemu-devel] Request for comment - dynamic VNC keyboard mapping
Date: Thu, 9 May 2019 15:02:58 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1

Hi Mario,

Sorry for the delayed answer.  As Daniel already said, any solution based
on how "-k <keymap>" works are too daunting to be maintained - there
are a lot of known keymaps to be aware of.

There's also the case of changing keymaps during the same session.
Speaking from experience: my laptop has a regular en-us keyboard, but
I also use it with an external Brazilian keyboard (which has the "Ç" key and
other punctuation keys). I happen to use some VMs in this hardware too.
It is not uncommon in my usage, in the same VNC session, a change in the
current keyboard layout en_us -> pt_br back and forth.

This is only possible because gtk-vnc does not care about my layout, but for
the keycodes being pressed. In my case, instead of sending the "Ç" symbol,
it would send the keycode of the 11th keyboard key located at the forth line.
The translation of keycode to keysym happens up to the OS layer.

gtk-vnc implements the RFB scancode extension that allows for the keycode to
be sent to VNC. This implementation isn't complicated to pull it off, as Daniel also mentioned in his reply. Many VNC clients already supports it. If you want
something to start from, I've implemented this extension in a browser VNC
client called noVNC a few years ago and I talked about it here:


You may also want to check Daniel's article about all this scancode/keycode/
keysym theory here:




On 5/3/19 8:47 AM, Mario wrote:
Hi all,

I have a question related to the VNC server keyboard settings. Currently the 
user of qemu has to decide before VM startup which language is used for VNC 
keyboard mapping. If no keyboard is configured, the en-us keyboard will be 
loaded for keysym to scancode conversion. A later reconfiguration of the 
keyboard is not possible at present.

So here is my idea:

The VNC Display context stores a list of loaded keyboards (like vs->vd->kbds). If no 
keyboard was specified, at least the default will be added to the list ("en-us"?). If 
a client connects, a copy of the (latest configured) pointer will be stored within the VncState 
structure. For any keys sent by this client the own keyboard reference will be used.

As (in theory) every VNC client may have a differen keyboard layout, in my 
point of view it makes sense to attach the keyboard used for keysym2scancode() 
to the client context rather to the VNC server context. However as (most 
likely) all clients do have the same keyboard mapping it would be an overhead 
to parse and store the mapping list each time the client connects.

Now in my idea two mechanisms may change the behavior of the client keyboard.

1.) a qmp command was sent, that sets /configures for a particular/default 
2.) within the VNC protocol a 

If the new keyboard was not already loaded and stored inside vs->vd->kbds , 
qemu initializes the new mapping. The affected VNC client (VncState) again stores the 
reference only.

This enables switching the keyboard layout during runtime as well as using 
multiple different keyboard layouts at atime.

Together with a corresponding patch I would also add a patch for libvncclient 
to have a reference implmentation for the integrated keyboard layout switching.

What are your thoughts about this kind of improvement?


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