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

[Daniel P. Berrange]

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.

Regards,
Daniel
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