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[Xen-devel] [PATCH qemu-xen 4.10 & 4.11] xen_disk: Disable file locking for the PV disk backend

Since QEMU 2.10 (or qemu-xen-4.10), qemu locks disk images to avoid
them been re-opened in a different qemu process.

With Xen, there are two issues:
- For HVM guests, a disk image can be open twice! One by the
  emulation driver, and one by the PV backend.
- During migration, the qemu process of the newly spawned domain may
  attempt to access the disk image before the domain been migrated
  and the qemu process are been completely destroyed.

Migration of HVM guest as been taken care of in libxl, but migration
of PV guest with qdisk and HVM guest attempting to access the PV disk
before unplugging the emulated disk are still an issue.

For these reasons, we don't want to have QEMU use a locking mechanism
with the PV backend.

This is already done by db9ff46eeb in QEMU upstream, or QEMU 4.0.

Affected version of QEMU are:
- qemu-xen of Xen 4.10 and 4.11
- QEMU 2.10, 2.11, 2.12, 3.0 and 3.1

Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>


Hi Olaf, Roger, Wei,

Instead of a fix in libxl (with the reverted patch "libxl: fix
migration of PV and PVH domUs with and without qemu"), what do you
think of this QEMU patch?  It is much simpler that trying to work
around the issue in libxl, and it fix one more problem that still

This patch is only for stable branches as Xen unstable already have a
qemu with a similar patch.

Can you give it a try with one of the affected qemu? (qemu-xen-4.10 or

 hw/block/xen_disk.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/hw/block/xen_disk.c b/hw/block/xen_disk.c
index e431bd89e8..cbff174a98 100644
--- a/hw/block/xen_disk.c
+++ b/hw/block/xen_disk.c
@@ -1084,11 +1084,18 @@ static int blk_connect(struct XenDevice *xendev)
         Error *local_err = NULL;
         QDict *options = NULL;
+        options = qdict_new();
         if (strcmp(blkdev->fileproto, "<unset>")) {
-            options = qdict_new();
             qdict_put_str(options, "driver", blkdev->fileproto);
+        /*
+         * It is necessary to turn file locking off as an emulated device
+         * may have already opened the same image file.
+         */
+        qdict_put_str(options, "file.locking", "off");
         /* setup via xenbus -> create new block driver instance */
         xen_pv_printf(&blkdev->xendev, 2, "create new bdrv (xenbus setup)\n");
         blkdev->blk = blk_new_open(blkdev->filename, NULL, options,
Anthony PERARD

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