RE: [PATCH v2 4/4] xen-blkback: support dynamic unbind/bind

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

> -----Original Message-----
> From: Roger Pau Monné <roger.pau@citrix.com>
> Sent: 11 December 2019 10:46
> To: Durrant, Paul <pdurrant@amazon.com>
> Cc: xen-devel@lists.xenproject.org; linux-kernel@vger.kernel.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 v2 4/4] xen-blkback: support dynamic unbind/bind
> 
> On Tue, Dec 10, 2019 at 11:33:47AM +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 fio --name=randwrite --ioengine=libaio --iodepth=16 \
> >   --rw=randwrite --bs=4k --direct=1 --size=1G --verify=crc32;
> >   done
> >
> > in a PV guest whilst running:
> >
> > while true;
> >   do echo vbd-$DOMID-$VBD >unbind;
> >   echo unbound;
> >   sleep 5;
> 
> Is there anyway to know when the unbind has finished? AFAICT
> xen_blkif_disconnect will return EBUSY if there are in flight
> requests, and the disconnect won't be completed until those requests
> are finished.

Yes, the device sysfs node will disappear when remove() completes.

> 
> >   echo vbd-$DOMID-$VBD >bind;
> >   echo bound;
> >   sleep 3;
> >   done
> >
> > in dom0 from /sys/bus/xen-backend/drivers/vbd to continuously unbind and
> > re-bind its system disk image.
> >
> > This is a highly useful feature for a backend module as it allows it to
> be
> > unloaded and re-loaded (i.e. updated) without requiring domUs to be
> halted.
> > This was also tested by running:
> >
> > while true;
> >   do echo vbd-$DOMID-$VBD >unbind;
> >   echo unbound;
> >   sleep 5;
> >   rmmod xen-blkback;
> >   echo unloaded;
> >   sleep 1;
> >   modprobe xen-blkback;
> >   echo bound;
> >   cd $(pwd);
> >   sleep 3;
> >   done
> >
> > in dom0 whilst running the same loop as above in the (single) PV guest.
> >
> > Some (less stressful) testing has also been done using a Windows HVM
> guest
> > with the latest 9.0 PV drivers installed.
> >
> > Signed-off-by: Paul Durrant <pdurrant@amazon.com>
> > ---
> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> > Cc: "Roger Pau Monné" <roger.pau@citrix.com>
> > Cc: Jens Axboe <axboe@kernel.dk>
> > Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> > Cc: Juergen Gross <jgross@suse.com>
> > Cc: Stefano Stabellini <sstabellini@kernel.org>
> >
> > v2:
> >  - Apply a sanity check to the value of rsp_prod and fail the re-attach
> >    if it is implausible
> >  - Set allow_rebind to prevent ring from being closed on unbind
> >  - Update test workload from dd to fio (with verification)
> > ---
> >  drivers/block/xen-blkback/xenbus.c | 59 +++++++++++++++++++++---------
> >  1 file changed, 41 insertions(+), 18 deletions(-)
> >
> > diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-
> blkback/xenbus.c
> > index e8c5c54e1d26..13d09630b237 100644
> > --- a/drivers/block/xen-blkback/xenbus.c
> > +++ b/drivers/block/xen-blkback/xenbus.c
> > @@ -181,6 +181,8 @@ static int xen_blkif_map(struct xen_blkif_ring
> *ring, grant_ref_t *gref,
> >  {
> >  	int err;
> >  	struct xen_blkif *blkif = ring->blkif;
> > +	struct blkif_common_sring *sring_common;
> > +	RING_IDX rsp_prod, req_prod;
> >
> >  	/* Already connected through? */
> >  	if (ring->irq)
> > @@ -191,46 +193,66 @@ static int xen_blkif_map(struct xen_blkif_ring
> *ring, grant_ref_t *gref,
> >  	if (err < 0)
> >  		return err;
> >
> > +	sring_common = (struct blkif_common_sring *)ring->blk_ring;
> > +	rsp_prod = READ_ONCE(sring_common->rsp_prod);
> > +	req_prod = READ_ONCE(sring_common->req_prod);
> > +
> >  	switch (blkif->blk_protocol) {
> >  	case BLKIF_PROTOCOL_NATIVE:
> >  	{
> > -		struct blkif_sring *sring;
> > -		sring = (struct blkif_sring *)ring->blk_ring;
> > -		BACK_RING_INIT(&ring->blk_rings.native, sring,
> > -			       XEN_PAGE_SIZE * nr_grefs);
> > +		struct blkif_sring *sring_native =
> > +			(struct blkif_sring *)ring->blk_ring;
> 
> I think you can constify both sring_native and sring_common (and the
> other instances below).

Yes, I can do that. I don't think the macros would mind.

> 
> > +		unsigned int size = __RING_SIZE(sring_native,
> > +						XEN_PAGE_SIZE * nr_grefs);
> > +
> > +		BACK_RING_ATTACH(&ring->blk_rings.native, sring_native,
> > +				 rsp_prod, XEN_PAGE_SIZE * nr_grefs);
> > +		err = (req_prod - rsp_prod > size) ? -EIO : 0;
> >  		break;
> >  	}
> >  	case BLKIF_PROTOCOL_X86_32:
> >  	{
> > -		struct blkif_x86_32_sring *sring_x86_32;
> > -		sring_x86_32 = (struct blkif_x86_32_sring *)ring->blk_ring;
> > -		BACK_RING_INIT(&ring->blk_rings.x86_32, sring_x86_32,
> > -			       XEN_PAGE_SIZE * nr_grefs);
> > +		struct blkif_x86_32_sring *sring_x86_32 =
> > +			(struct blkif_x86_32_sring *)ring->blk_ring;
> > +		unsigned int size = __RING_SIZE(sring_x86_32,
> > +						XEN_PAGE_SIZE * nr_grefs);
> > +
> > +		BACK_RING_ATTACH(&ring->blk_rings.x86_32, sring_x86_32,
> > +				 rsp_prod, XEN_PAGE_SIZE * nr_grefs);
> > +		err = (req_prod - rsp_prod > size) ? -EIO : 0;
> >  		break;
> >  	}
> >  	case BLKIF_PROTOCOL_X86_64:
> >  	{
> > -		struct blkif_x86_64_sring *sring_x86_64;
> > -		sring_x86_64 = (struct blkif_x86_64_sring *)ring->blk_ring;
> > -		BACK_RING_INIT(&ring->blk_rings.x86_64, sring_x86_64,
> > -			       XEN_PAGE_SIZE * nr_grefs);
> > +		struct blkif_x86_64_sring *sring_x86_64 =
> > +			(struct blkif_x86_64_sring *)ring->blk_ring;
> > +		unsigned int size = __RING_SIZE(sring_x86_64,
> > +						XEN_PAGE_SIZE * nr_grefs);
> > +
> > +		BACK_RING_ATTACH(&ring->blk_rings.x86_64, sring_x86_64,
> > +				 rsp_prod, XEN_PAGE_SIZE * nr_grefs);
> > +		err = (req_prod - rsp_prod > size) ? -EIO : 0;
> 
> This is repeated for all ring types, might be worth to pull it out of
> the switch...
> 

I did wonder about that... I'll do in v3.

> >  		break;
> >  	}
> >  	default:
> >  		BUG();
> >  	}
> > +	if (err < 0)
> > +		goto fail;
> 
> ...and placed here instead?

Indeed.

  Cheers,
    Paul

> 
> Thanks, Roger.