Re: RFC: PV vfb's and the handling of global vnc options

[Ian Campbell <ian.campbell@citrix.com>]

On Mon, 2015-11-09 at 15:03 +0000, George Dunlap wrote:

Adding other tools maintainers.

> So I had a user report that he couldn't get the vnclisten option to
> work.  It turns out he was using a PV guest, and had the following in
> his config file:
> 
> vfb=[ 'type=vnc' ]
> vnclisten='0.0.0.0'
> 
> After digging around in the code, it turns out that the following is
> accepted for PV guests:
> 
> vnc=1
> vnclisten='0.0.0.0'
> 
> ...on the condition that there be *no* vfb option.  If you set the vfb
> option, then the top-level vnc entries are completely ignored.
> 
> The following also works (as one might expect):
> 
> vfb=[ 'type=vnc,vnclisten=0.0.0.0' ]
> 
> The man page is a bit unclear here; it says:
> 
> If "Emulated VGA Graphics Device" options are used in a PV guest
> configuration, xl will pick up vnc, vnclisten, vncpasswd, vncdisplay,
> vncunused, sdl, opengl and keymap to construct paravirtual framebuffer
> device for the guest.
> 
> I see two options:
> 
> 1. Make vfb take defaults from the top-level vnc options
> 
> 2. Clarify the man page to make it clear that top-level vnc options
> will only have an effect if no vfb is specified.
> 
> I'm of two minds which one is best.  At the moment I'm leaning towards
> #2, possibly adding a warning if it finds both vfb and toplevel vnc
> options, saying that it's ignoring the second.

One problem with #1 is that for some options (vncdisplay) having a global
default for potentially multiple vfb=[] stanzas would be wrong and a bit
confusing (although perhaps no more so than the current behaviour).

I suppose you could limit some or (better) all of the globals to only being
used for the first entry in the array (but I bet the code gets quite funky
in that case).

You'd probably want the warning you mention in #2 even in the #1 case.

Like you I'm not really sure.

A third option just to muddy the waters:

3. If any global options are given then a new vfb is pushed onto the front
of any array given by vfb=[], i.e. those vfbs implicitly go from devices
0..N to 1..N+1.

Ian.



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