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

Durrant, Paul
2019-12-11T10:52:51Z

> -----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.