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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