[Spice-devel] agent server code migration code is broken / non existent

[Alon Levy]

On Mon, Apr 04, 2011 at 01:34:22PM +0200, Hans de Goede wrote:
> Hi All,
> 
> While testing the agent selection patches I found a bug in
> spice-gtk which in turn triggered a bug in spice-server wrt agent
> handling. As demonstrated by the recent server agent patch set
> I did this led to me finding another bug, etc...
> 
> While looking into this I've also taken a quick look at
> the saving / restoring of agent related state (basically
> the VDIPortState contents)  over a migration. There
> is code in server/reds.c to handle this, but it only
> does so in the seamless migration case! Since we don't
> support seamless migration atm, this means that effectively
> all state in VDIPortState does not get kept over a
> migration.
> 
> This means that:
> 
> * If the agent is halfway through sending an agent message
> chunk when we switch to the new vm, we will see the data
> halfway through the chunk as a chunk header, declare it
> invalid and disconnect the agent (not good (tm)).
> 
> * If there was any data from the client queued to be
> delivered to the agent when we switch to the new vm, all
> queued data is lost (not good (tm)).
> 
> Also I must say I don't understand why in the seamless case
> the *servers* agent state (such as it state wrt parsing a chunk
> being send from the agent, which could be a chunk destined for
> the server) is being send to the new through the client at all,
> this makes no sense, this should be using qemu's normal
> migration mechanisms (*).

There is also the possibly related issue of lack (and opposition) to bi
directional migration protocol for qemu.

> 
> So long story short someone (hint preferably not me I would like
> to get back to usb work again) needs to look into properly
> implementing migration for the vdiportstate (and maybe also
> other mainchannel related state).
> 
> Regards,
> 
> Hans
> 
> *) I believe this may be an example of "once you have a hammer
> everything looks like a nail" example, iow I have the theory
> this was implemented through seamless, because back when it
> was implemented seamless migration was seen as the solution to all
> migration related problems.
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