Re: [Xen-devel] [PATCH] libxl: create backend/ xenstore dir for driver domains

[Ian Jackson <ian.jackson@citrix.com>]

Adding Roger to the CC.

Marek Marczykowski-Górecki writes ("Re: [PATCH] libxl: create backend/ xenstore dir for driver domains"):
> On Mon, Jan 06, 2020 at 02:20:46PM +0000, Ian Jackson wrote:
> > Marek Marczykowski-Górecki writes ("[PATCH] libxl: create backend/ xenstore dir for driver domains"):
> > > Cleaning up backend xenstore entries is a responsibility of the backend.
> > > When backend lives outside of dom0, the domain needs proper permissions
> > > to do it. Normally it is given permission to remove the device dir
> > > itself, but not the dir containing it (named after frontend ID). After a
> > > whole those empty leftover directories accumulate to the point xenstore
> > > returning E2BIG on listing them.
> > > 
> > > Fix this by giving backend domain write access also to backend/
> > > directory itself when c_info->driver_domain option is set. The code
> > > removing relevant dir is already there (just lacked permissions to do so).
> > > 
> > > Note this also allows the backend domain to create new entries,
> > > pretending to host backend devices it don't have. But since libxl uses
> > > /libxl/ xenstore dir for this information (still outside of backend
> > > domain control), this shouldn't be an issue.
> > 
> > This seems quite hazardous to me.  The reasoning you use to show that
> > this iws OK seems fragile, and in general it doesn't feel right to
> > give the particular backend such wide scope.
> > 
> > Can we find another way to address this problem ?  I think the
> > containing directory should be removed by the toolstack.  Why is this
> > difficult ?  (I presume there is a reason or you would have done it
> > that way...)
> 
> It was done this way previously and caused issues, see this commit:
> 
> commit 546678c6a60f64fb186640460dfa69a837c8fba5
> Author: Roger Pau Monne <roger.pau@citrix.com>
> Date:   Wed Sep 23 12:06:56 2015 +0200
> 
>     libxl: fix the cleanup of the backend path when using driver domains

Thanks.

>     With the current libxl implementation the control domain will
>     remove both the frontend and the backend xenstore paths of a
>     device that's handled by a driver domain. This is incorrect,
>     since the driver domain possibly needs to access the backend
>     path in order to perform the disconnection and cleanup of the
>     device.
>     
>     Fix this by making sure the control domain only cleans the
>     frontend path, leaving the backend path to be cleaned by the
>     driver domain. Note that if the device is not handled by a
>     driver domain the control domain will perform the removal of
>     both the frontend and the backend paths.

Hmm.  I see my Ack on that.  Nevertheless maybe it is wrong.

Looking at it afresh, I think maybe the right answer is:

 * If the driver domain is expected to be working properly, the
   toolstack should wait for the driver domain to complete the device
   shutdown, before removing the backend node.  Indeed, the toolstack
   ought to wait for this before actually destroying the guest in Xen,
   by the usual logic for clean domain shutdown.

 * There needs to be a way to deal with a broken/unresponsive driver
   domain.  That will involve not waiting for the backend so must
   involve simply deleting the backend from xenstore.

Is the distinction here between "xl shutdown" and "xl destroy", on the
actual guest domain, good enough ?  Hopefully if the driver domain
sees the backend directory simply vanish it can destructively tear
everything down ?

Ian.

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