Re: Qemu, VNC and non-US keymaps

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, May 11, 2020 at 05:29:48PM +0200, B3r3n wrote:
> Hello Daniel,
> 
> > There is no mention here of what VNC client program is being used, which
> > is quite important, as key handling is a big mess in VNC.
> I tested with TightVNC & noVNC through Apache. Both behaves the same. I did
> not tested Ultr@VNC.

AFAIK, neithe TightVNC nor Ultra@VNC support the scancode extension.

noVNC does, for most modern browsers, so it might work if you remove
the -k arg from QEMU.

> > The default VNC protocol passes X11 keysyms over the wire.
> > 
> > The remote desktop gets hardware scancodes and turns them into keysyms,
> > which the VNC client sees. The VNC client passes them to the VNC server
> > in QEMU, which then has to turn them back into hardware scancodes. This
> > reverse mapping relies on knowledge of the keyboard mapping, and is what
> > the "-k fr" argument tells QEMU.
> > 
> > For this to work at all, the keymap used by the remote desktop must
> > match the keymap used by QEMU, which must match the keymap used by
> > the guest OS.  Even this is not sufficient though, because the act
> > of translating hardware scancodes into keysyms is *lossy*. There is
> > no way to reliably go back to hardware scancodes, which is precisely
> > what QEMU tries to do - some reverse mappings will be ambiguous.
> Yes, I saw that topic passing by. Looks messy with all these interferences...
> 
> > Due to this mess, years ago (over a decade) QEMU introduced a VNC
> > protocol extension that allows for passing hardware scancodes over
> > the wire.
> I guess I also crossed something about this on Internet.
> Are you talking of the RFB protocol ?

Yes, RFB protocol is the technical name for the VNC wire protocol.

> > With this extension, the VNC client gets the hardware scancode
> > from the remote desktop, and passes it straight to the VNC server,
> > which passes it straight to the guest OS, which then applies the
> > localized keyboard mapping.   This is good because the localized
> > keyboard mapping conversion is now only done once, in the guest
> > OS.
> > 
> > To make use of this protocol extension to VNC, you must *NOT*
> > pass any "-k" arg to QEMU, and must use a VNC client that has
> > support for this protocol extension.  The GTK-VNC widget supports
> > this and is used by virt-viewer, remote-viewer, virt-manager,
> > GNOME Boxes, Vinagre client applications.  The TigerVNC client
> > also supports this extension.
> So if I read you, if the client "enforce" this protocol (supposed RFB), Qemu
> will automatically uses it as well ?

The client should automatically activate the extension if QEMU advertizes
it, and QEMU advertizes it if you remove the -k arg.

> Removing -k option is great to me if it works, since user will have its own
> mapping and these are international :-)



> > To summarize, my recommendation is to remove the "-k" arg entirely,
> > and pick a VNC client that supports the scancode extension.
> For now I am using TightVNC & noVNC. noVNC is precious since it widens the
> user world, removing any client software constraint.

As above, noVNC ought to support the extension.

> 
> > It is possible there might be a genuine bug in QEMU's 'fr' keymap
> > that can be fixed to deal with AltGr problems. Personally though I
> > don't spend time investigating these problems, as the broad reverse
> > keymapping problem is unfixable. The only sensible option is to take
> > the route of using the VNC hardware scancode extension. It is notable
> > that SPICE learnt from VNC's mistake and used hardware scancodes from
> > the very start.
> 
> This was another path I intend to follow : using SPICE and a "noSPICE"
> client if VNC was too painful.
> If I understand you, using SPICE could also solve the issue ?
> 
> Many thanks for your inputs...

Regards,
Daniel
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