Re: [Qemu-devel] [PATCH] spice: add display & head options

[Marc-André Lureau]

Hi

On Tue, Feb 21, 2017 at 2:04 PM Gerd Hoffmann <address@hidden> wrote:

>   Hi,
>
> > It's always 0, except with virtio.
> >
> > But I think virtio linux driver multi-monitor code would work with a
> > single qemu head (like qxl, fwiw last time I checked, multi-monitor
> > virtio+spice worked, with 3d too).
>
> > It seems to me the qemu "head" is quite different (lower-level) from
> > spice "head".
>
> Well.  spice/qxl supports two different ways to do multihead ...
>
> The first way is one display channel per head.  Which maps nicely to
> qemu heads.  With qxl you'll use one qxl-vga device for the primary
> display and one (or more) qxl devices for the secondary displays.  I
> think multihead with windows guests still works that way, but xorg
> support for that (xinerama mode) is pretty bad.  IIRC it needs manual
> configuration, could also be recent xorg versions don't support it any
> more.  Dunno where we stand with wayland.
>
> The second way is a single display channel with a single, big surface
> for all heads.  Then -- with spice-agent help -- guest and client
> negotiate which region of the big display gets scanned out to which
> head.
>
> Multihead spice with virtio-gpu fundamentally works like the first way,
> except that we have a single device with multiple heads (and display
> channels) instead of multiple devices which have a single head each.
>

ok


>
> The use case I see for this is not so much multihead configuration.
> With multiple display devices in your system, for example one intel-vgpu
> and one emulated display device one could export only one of the two via
> spice.
>

My understanding is that "head" will not work correctly with QXL (only head
0, but can be a multi-head QXL), but should do what you expect with virtio.

thanks
-- 
Marc-André Lureau