[Xen-changelog] [qemu-xen stable-4.10] xen_disk: Disable file locking for the PV disk backend

[patchbot@xxxxxxx]

commit 8acabec966263f90ad493e4af2642947c0c43d23
Author:     Anthony PERARD <anthony.perard@xxxxxxxxxx>
AuthorDate: Wed May 22 15:09:32 2019 +0100
Commit:     Anthony PERARD <anthony.perard@xxxxxxxxxx>
CommitDate: Fri Jun 7 10:59:05 2019 +0100

    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>
---
 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 536e2ee735..74ac01b960 100644
--- a/hw/block/xen_disk.c
+++ b/hw/block/xen_disk.c
@@ -1090,11 +1090,18 @@ 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,
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#stable-4.10

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