Re: [Xen-users] Xen handling of graphics card

[Ralph Passgang <ralph@xxxxxxxxxxxxx>]

Am Freitag, 13. Januar 2006 00:10 schrieb Jayesh Salvi:
> > > > Xen offers vga emulation through a VNC backend.  The
> > >
> > > Xen-Windows video
> > >
> > > > driver you speak of would probably just be a VNC driver that turns
> > > > graphic commands directly into VNC encoding rather than
> > >
> > > traversing the
> > >
> > > > VGA emulation layer.  Perhaps this beast already exists?
> > >
> > > Maybe this
> > >
> > > > is
> > >
> > > You mean that Xen has it own VNC server embedded?
> > > Or does it depend on an external one managed by the OS?
> >
> > The VNC server is managed by Dom0.
>
>  I am confused by this discussion. As I understand it, VNC server is a tool
> which runs irrespective of the presence of any graphics card. I ran a VNC
> server inside domU and connected to it from a vnc client in dom0. I am sure
> vice-versa is also possible. And since VNC servers are available for all
> platforms (including windows and MAC), in VT environment when windows will
> be run in domU, you can run a VNC server inside guest  Windows and connect
> to it from dom0 or any other machine (provided there is network connection
> between these virtual machines).
>
> I don't know how VNC server/client renders images or marshals them over
> wire (may be they have some in built emulated stuff), but I am pretty sure
> they don't use native graphics card. That is why even though VNC will solve
> the problems of GUI to guest OSes, it won't solve problems like playing
> high end games in guest windows instance.

I think you will have a virtual pci device in your domU's windows. This 
virtual vga cards looks like a std. vga adapter to your windows. The virtual 
vga adapter doesn't render anything to a vga or dvi port (because there is no 
such port on a virtual pci device *g*), but will make the windows session 
available through vnc (don't know on dom0 or anyhow on domUs IP). So you will 
be able to see windows booting, even if at this stage no vnc server could 
even run. I am not sure what about text mode, but that's probably I just 
because I never saw a vnc server providing a terminal or something like that 
(for example dos, linux, ...)

Never though about it. Should I be able to see vnc from a native linux running 
on a vt system? even in textmode? I guess so... :)

Hmmm, I think that's a quite easy but beautiful way to solve the vga adapter 
problem for windows and all other domainUs running natively on a VT cpu. A 
standard vga adapter is supported in every pc os.

that brings up another question... as I read before, the emulated devices are 
producing (at the moment noticeable) overhead. Is the vga adapter also 
producing noticeable overhead, even if no vnc client is connected? If the 
adapter always produces overhead, does it make sense to put the "monitor" in 
standby after 1 minute of inactivity in the windows vga adapter settings? 
Would that produce less overhead then to have always some data going to your 
vga adapter?

--Ralph

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