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[libvirt] [PATCH v3 3/9] qemuMigrationDriveMirror: Pass disk format to qemu

When playing with disk migration lately, I've noticed this warning in
domain logs:

WARNING: Image format was not specified for 'nbd://masina:49153/drive-virtio-disk0' and probing guessed raw.
         Automatically detecting the format is dangerous for raw images, write operations on block 0 will be restricted.
         Specify the 'raw' format explicitly to remove the restrictions.

So I started digging into qemu source code to see what has triggered
the warning. I'd expect qemu to know formats of guest's disks since we
tell them on command line. This lead me to qmp_drive_mirror() where
the following can be found:

    if (!has_format) {
        format = mode == NEW_IMAGE_MODE_EXISTING ? NULL : bs->drv->format_name;

So, format is automatically initialized from the disk iff mode !=
"existing". Unfortunately, in migration we are tied to use this mode
(NBD doesn't support creating new images). Therefore the only way to
avoid this warning is to pass format. The format that libvirt thinks
should be in sync with qemu anyway.

Signed-off-by: Michal Privoznik <mprivozn redhat com>
 src/qemu/qemu_migration.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/qemu/qemu_migration.c b/src/qemu/qemu_migration.c
index 32c43f3..279b43f 100644
--- a/src/qemu/qemu_migration.c
+++ b/src/qemu/qemu_migration.c
@@ -1943,6 +1943,7 @@ qemuMigrationDriveMirror(virQEMUDriverPtr driver,
     for (i = 0; i < vm->def->ndisks; i++) {
         virDomainDiskDefPtr disk = vm->def->disks[i];
         qemuDomainDiskPrivatePtr diskPriv = QEMU_DOMAIN_DISK_PRIVATE(disk);
+        const char *format;
         int mon_ret;
         /* skip shared, RO and source-less disks */
@@ -1950,6 +1951,9 @@ qemuMigrationDriveMirror(virQEMUDriverPtr driver,
+        if (disk->src->format > 0)
+            format = virStorageFileFormatTypeToString(disk->src->format);
         if ((virAsprintf(&diskAlias, "%s%s",
@@ -1967,7 +1971,7 @@ qemuMigrationDriveMirror(virQEMUDriverPtr driver,
         mon_ret = qemuMonitorDriveMirror(priv->mon, diskAlias, nbd_dest,
-                                         NULL, speed, 0, 0, mirror_flags);
+                                         format, speed, 0, 0, mirror_flags);
         if (qemuDomainObjExitMonitor(driver, vm) < 0 || mon_ret < 0) {
             qemuBlockJobSyncEnd(driver, vm, disk, NULL);

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