Re: [Qemu-devel] [Spice-devel] [PATCH] RFC: qxl: allow to specify head limit to qxl driver

[Christophe Fergeau]

On Tue, Jun 09, 2015 at 10:50:48AM +0100, Daniel P. Berrange wrote:
> On Tue, Jun 09, 2015 at 09:49:44AM +0100, Frediano Ziglio wrote:
> > This patch allow to limit number of heads using qxl driver. By default
> > qxl driver is not limited on any kind on head use so can decide to use
> > as much heads.
> > 
> > libvirt has this as a video card parameter (actually set to 1 but not
> > used). This parameter will allow to limit setting a use can do (which
> > could be confusing).
> > 
> > This patch rely on some change in spice-protocol which are not still
> > accepted. See
> > http://lists.freedesktop.org/archives/spice-devel/2015-June/020160.html.
> > 
> > Main question and stop over are parameter name. Consider that this
> > parameter is actually more a hint to drivers. I'm looking anyway to
> > a way to enforce this in spice-server.
> 
> What is the actual benefit of being able to limit the number of
> heads in QXL / SPICE ?
> 
> I know libvirt has the 'heads' attribute for specifying the number
> of video outputs. This is used in hypervisors which only support a
> fixed number of outputs and need up-front configuration. Since
> QXL/SPICE can dynamically enable/disable heads, there is no need to
> support this libvirt configuration attribute - it is better from a
> usability POV to have the head count totally dynamic, than to add
> a fixed limit.
> 
> IOW, unless there's some need to use this limit in order to go
> above the 16 head maximum the code currently has, I think adding
> this manual limit to QXL/SPICE is a step backwards really. It
> feels like a feature in search of a purpose IMHO.

The head count is indeed dynamic with SPICE, but the available video RAM
is fixed after QEMU startup, and if you start trying to fit 4 full HD
heads in 16MB of VRAM, you'll get weird failures. If you know you are
only going to use one screen, it's nice to be able to allocate a small
amount of VRAM and to enforce the maximum number of heads for the amount
of VRAM you have assigned to the VM.
remote-viewer also gets a 'display' menu with the number of available
displays listed in it. I can imagine some people/admins wanted to
mandate that only N displays are going to be shown here to avoid user
confusion.

So while we should keep defaulting to the current dynamic behaviour,
a way to optionally limit the number of available heads is useful in
some situations

Christophe

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