RE: [PATCH 4/4] xen-blkback: support dynamic unbind/bind

["Durrant, Paul" <pdurrant@amazon.com>]

> -----Original Message-----
> From: Roger Pau Monné <roger.pau@citrix.com>
> Sent: 09 December 2019 12:17
> To: Durrant, Paul <pdurrant@amazon.com>
> Cc: linux-kernel@vger.kernel.org; xen-devel@lists.xenproject.org; Konrad
> Rzeszutek Wilk <konrad.wilk@oracle.com>; Jens Axboe <axboe@kernel.dk>;
> Boris Ostrovsky <boris.ostrovsky@oracle.com>; Juergen Gross
> <jgross@suse.com>; Stefano Stabellini <sstabellini@kernel.org>
> Subject: Re: [PATCH 4/4] xen-blkback: support dynamic unbind/bind
> 
> On Thu, Dec 05, 2019 at 02:01:23PM +0000, Paul Durrant wrote:
> > By simply re-attaching to shared rings during connect_ring() rather than
> > assuming they are freshly allocated (i.e assuming the counters are zero)
> > it is possible for vbd instances to be unbound and re-bound from and to
> > (respectively) a running guest.
> >
> > This has been tested by running:
> >
> > while true; do dd if=/dev/urandom of=test.img bs=1M count=1024; done
> >
> > in a PV guest whilst running:
> >
> > while true;
> >   do echo vbd-$DOMID-$VBD >unbind;
> >   echo unbound;
> >   sleep 5;
> >   echo vbd-$DOMID-$VBD >bind;
> >   echo bound;
> >   sleep 3;
> >   done
> 
> So this does unbind blkback while leaving the PV interface as
> connected?
> 

Yes, everything is left in place in the frontend. The backend detaches from the ring, closes its end of the event channels, etc. but the guest can still send requests which will get serviced when the new backend attaches.

  Paul