Re: [Qemu-devel] [Spice-devel] [RFC PATCH spice 1/2] QXL interface: add functions to identify monitors in the guest

[Gerd Hoffmann <kraxel@redhat.com>]

> > So, if I remember correctly, Gerd recommended returning this value from
> > the function. But I think it needs more explanation here. What exactly
> > is a "monitor_id" supposed to represent? It is not used in your follow-
> > up qemu patch so it's hard to tell.
> 
> It's supposed to be the actual monitor_id that we use in SPICE to
> identify the monitors. I've just spent quite some time looking up where
> the monitor_id actually comes from, and it's actually set all the way
> down in the driver (either xf86-video-qxl or the KMS driver in the
> kernel) and passed through the monitors_config functions to SPICE
> server.

How does all this monitors_config work currently in case multiple
display channels are present?

There is the QXLInterface->client_monitors_config() callback.

There is the spice_qxl_monitors_config_async() function.

Both are linked to a display channel.  The server/client messages go
through the main channel though ...

So what happens if a message from the client arrives?  Is that
broadcasted as-is to all display channels?  The current qemu code
(interface_client_monitors_config in spice-display.c, which is used with
virtio-gpu) simply uses the head as index into the array, so it looks
like spice-server doesn't do any filtering here (like only filling in
the monitors which belong to the display channel).

spice_qxl_monitors_config_async() is never called with virtio-gpu,
except when opengl is enabled.  In that case qemu simply sets
QXLMonitorsConfig->count to 1 and fills in QXLMonitorsConfig->head[0]
(see qemu_spice_gl_monitor_config in spice-display.c).  Which would be
ok in case spice-server merges the configs from all channels before
sending it off to the client.  Not sure this actually happens ...

> Interstingly enough, that seems to be the ID we want to have in the
> device_display_id attribute.  I expect (still need to look that up, I'm
> out of time right now) that for virtio-gpu the ID is a bit different,

Keep in mind that multiple display devices don't really work right now,
and possibly we need to fix not only spice but qemu too.

> And yeah, I didn't use the id in the QEMU patches, as I didn't know
> how, I expect Gerd to have some grand plans for it :)

IIRC the latest plan was to just keep things as is, plan with one
channel per monitor for all future things, and just not support
one-qxl-device-multihead in combination with multiple display channels.

Is that correct?

I don't think we need the monitors_id in qemu then, qemu can simply use
the channel_id (except for the legacy qxl case).

cheers,
  Gerd