xen-blkback: support dynamic unbind/bind

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>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/block/xen-blkback/xenbus.c

@@ -190,6 +190,9 @@ static int xen_blkif_map(struct xen_blkif_ring *ring, grant_ref_t *gref,
 {
 	int err;
 	struct xen_blkif *blkif = ring->blkif;
+	const struct blkif_common_sring *sring_common;
+	RING_IDX rsp_prod, req_prod;
+	unsigned int size;
 
 	/* Already connected through? */
 	if (ring->irq)
@@ -200,46 +203,62 @@ 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;
+
+		BACK_RING_ATTACH(&ring->blk_rings.native, sring_native,
+				 rsp_prod, XEN_PAGE_SIZE * nr_grefs);
+		size = __RING_SIZE(sring_native, XEN_PAGE_SIZE * nr_grefs);
 		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;
+
+		BACK_RING_ATTACH(&ring->blk_rings.x86_32, sring_x86_32,
+				 rsp_prod, XEN_PAGE_SIZE * nr_grefs);
+		size = __RING_SIZE(sring_x86_32, XEN_PAGE_SIZE * nr_grefs);
 		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;
+
+		BACK_RING_ATTACH(&ring->blk_rings.x86_64, sring_x86_64,
+				 rsp_prod, XEN_PAGE_SIZE * nr_grefs);
+		size = __RING_SIZE(sring_x86_64, XEN_PAGE_SIZE * nr_grefs);
 		break;
 	}
 	default:
 		BUG();
 	}
 
+	err = -EIO;
+	if (req_prod - rsp_prod > size)
+		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)
@@ -1131,7 +1150,8 @@ 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)