Re: [Xen-devel] [PATCH 12 of 14 v4] libxl: set frontend status to 6 on domain destroy

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Roger Pau Monné writes ("Re: [Xen-devel] [PATCH 12 of 14 v4] libxl: set 
frontend status to 6 on domain destroy"):
> 2011/12/13 Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>:
> > So here, the kernel or backendd start racing, and you hope that they
> > win the race and close the device before ...
> 
> From my experience in NetBSD, the kernel only closes the device when
> it's frontend state is set to 6, since we destroy the domain, it is
> unable to set the status to 6, and thus the kernel doesn't detach the
> devices.

This is no good, because you need to be able to force unplug a device
without guest cooperation, and the frontend state is controlled by the
guest.  So I think there is a NetBSD kernel bug in this area.

It needs to be possible to tell the backend to shut down without
regard to the frontend state.

> I've added some libxl__wait_for_device_state logic here, to
> assure the backend state is set to 6 before trying to execute hotplug
> scripts. The truth is that I had it in previous versions of my patch,
> but it seems the kernel always switches contexts and detaches the
> devices before executing hotplug scripts (it might just be luck).

Well that's very nice but of course not reliable.

> Also this patch speeds domain destruction a lot (which is also quite
> slow under Linux from what I saw).

You're saying that not waiting for the backends to close speeds up
domain destruction ?  Where is the time going ?

Ian.

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