> Dear VDSM developers,
> 
> I'm working on a patch that allows running a VM with multiple graphics
> framebuffers. This is handy when you want to run a VM with both SPICE and
> VNC. It's a 3.4 feature and it will certainly need a change in vdsm.
> Here is a list of changes in VDSM that are needed for this funcionality:
> a, Sending graphics/video (engine->vdsm)
>  - currently we send two things:
>   1, "display" value (qxl/vnc [wat])
>     - vdsm uses this for determining if the graphics server is SPICE or VNC
>     - this attribute is not really correct - it mixes up semantics of graphic
>       framebuffer and videocard together. I believe this attribute should
>       only
>       contain information about the graphics ('spice', 'vnc' or 'spice,vnc'
>       if
>       you want both). if this the case, do you think we should rename the
>       attribute
>       to, let's say, 'graphics'? Is it even possible with regard to backward
>       compatibility? or should I reuse 'display' attribute?
>   2, video device (json representation of the video card) - this is correct

In order to keep backwards compatibility after rename, engine would have to
send different command to newer/older VDSMs as older VDSM has no code to 
understand
different keyword - I'd say keep the name "display".

> b, Reporting graphics ports (vdsm->engine)
>  - currently we report 2 graphic ports ('displayPort' and
>  'secureDisplayPort')
>  - if we want multiple framebuffers, we must report more ports (for VNC and
>    SPICE together that would mean 3 ports (2 for spice, one for vnc).
>  - there are two possible solutions for this:
>   1, ditch 'displayPort' and 'secureDisplayPort' and add new 'spicePort',
>      'spiceSecurePort', 'vncPort' fields or some kind of two level dict:
>         { protocol -> secured/unsecured -> portNumber }
>   2, keep 'displayPort' and 'secureDisplayPort' and introduce new
>   'additionalDisplayPort'
>      This would be friendlier to backward compatibility, but it's extremely
>      ugly because of unclear semantics of the fields (in case of SPICE+VNC
>      'displayPort' and 'secureDisplayPort' would be related to SPICE,
>     'additionalDisplayPort' would be the VNC port. In case of VNC only, the
>     'displayPort' would be suddendly VNC port... ewww).

Same difficulty as previous case but on engine side: unless older engine can
process new keywords, our best and only backwards compatible choice is adding
a new field.

Also what won't remain unaffected is migration: migrating VM with multiple
framebuffers to older VDSM will afaik require patching the config causing
VM to only run with one framebuffer, possibly losing the information that
another was attached (in case of another migration to new VDSM).

Martin
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