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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