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

Paul Durrant
2019-12-10T11:34:44Z

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;
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;
+ 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;
break;
}
default:
BUG();
}
+ if (err < 0)
+ goto fail;

err = bind_interdomain_evtchn_to_irqhandler(blkif->domid, evtchn,
xen_blkif_be_int, 0,
"blkif-backend", ring);
- if (err < 0) {
- xenbus_unmap_ring_vfree(blkif->be->dev, ring->blk_ring);
- ring->blk_rings.common.sring = NULL;
- return err;
- }
+ if (err < 0)
+ goto fail;
ring->irq = err;

return 0;
+
+fail:
+ xenbus_unmap_ring_vfree(blkif->be->dev, ring->blk_ring);
+ ring->blk_rings.common.sring = NULL;
+ return err;
}

static int xen_blkif_disconnect(struct xen_blkif *blkif)
@@ -1121,7 +1143,8 @@ static struct xenbus_driver xen_blkbk_driver = {
.ids = xen_blkbk_ids,
.probe = xen_blkbk_probe,
.remove = xen_blkbk_remove,
- .otherend_changed = frontend_changed
+ .otherend_changed = frontend_changed,
+ .allow_rebind = true,
};

int xen_blkif_xenbus_init(void)
--
2.20.1