[Qemu-devel] [PULL 06/48] block: Reject writethrough mode except at the root

[Kevin Wolf <kwolf@redhat.com>]

Writethrough mode is going to become a BlockBackend feature rather than
a BDS one, so forbid it in places where we won't be able to support it
when the code finally matches the envisioned design.

We only allowed setting the cache mode of non-root nodes after the 2.5
release, so we're still free to make this change.

The target of block jobs is now always opened in a writeback mode
because it doesn't have a BlockBackend attached. This makes more sense
anyway because block jobs know when to flush. If the graph is modified
on job completion, the original cache mode moves to the new root, so
for the guest device writethough always stays enabled if it was
configured this way.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 block.c                    |  7 ++++
 blockdev.c                 | 19 ++++++++-
 tests/qemu-iotests/142     | 50 +++++++++++------------
 tests/qemu-iotests/142.out | 98 +++++++++-------------------------------------
 4 files changed, 68 insertions(+), 106 deletions(-)

diff --git a/block.c b/block.c
index 5da7bb2..d319f86 100644
--- a/block.c
+++ b/block.c
@@ -974,6 +974,13 @@ static int bdrv_open_common(BlockDriverState *bs, BdrvChild *file,
 
     /* Apply cache mode options */
     update_flags_from_options(&bs->open_flags, opts);
+
+    if (!bs->blk && (bs->open_flags & BDRV_O_CACHE_WB) == 0) {
+        error_setg(errp, "Can't set writethrough mode except for the root");
+        ret = -EINVAL;
+        goto free_and_fail;
+    }
+
     bdrv_set_enable_write_cache(bs, bs->open_flags & BDRV_O_CACHE_WB);
 
     /* Open the image, either directly or using a protocol */
diff --git a/blockdev.c b/blockdev.c
index 914b526..0f85ecd 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1771,6 +1771,12 @@ static void external_snapshot_prepare(BlkActionState *common,
         flags |= BDRV_O_NO_BACKING;
     }
 
+    /* There is no BB attached during bdrv_open(), so we can't set a
+     * writethrough mode. bdrv_append() will swap the WCE setting so that the
+     * backing file becomes unconditionally writeback (which is what backing
+     * files should always be) and the new overlay gets the original setting. */
+    flags |= BDRV_O_CACHE_WB;
+
     assert(state->new_bs == NULL);
     ret = bdrv_open(&state->new_bs, new_image_file, snapshot_ref, options,
                     flags, errp);
@@ -2515,6 +2521,7 @@ void qmp_blockdev_change_medium(const char *device, const char *filename,
     BlockBackend *blk;
     BlockDriverState *medium_bs = NULL;
     int bdrv_flags, ret;
+    bool writethrough;
     QDict *options = NULL;
     Error *err = NULL;
 
@@ -2533,6 +2540,12 @@ void qmp_blockdev_change_medium(const char *device, const char *filename,
     bdrv_flags &= ~(BDRV_O_TEMPORARY | BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING |
         BDRV_O_PROTOCOL);
 
+    /* Must open the image in writeback mode as long as no BlockBackend is
+     * attached. The right mode can be set as the final step after changing the
+     * medium. */
+    writethrough = !(bdrv_flags & BDRV_O_CACHE_WB);
+    bdrv_flags |= BDRV_O_CACHE_WB;
+
     if (!has_read_only) {
         read_only = BLOCKDEV_CHANGE_READ_ONLY_MODE_RETAIN;
     }
@@ -2590,6 +2603,8 @@ void qmp_blockdev_change_medium(const char *device, const char *filename,
         goto fail;
     }
 
+    bdrv_set_enable_write_cache(medium_bs, !writethrough);
+
     qmp_blockdev_close_tray(device, errp);
 
 fail:
@@ -3215,7 +3230,7 @@ static void do_drive_backup(const char *device, const char *target,
         goto out;
     }
 
-    flags = bs->open_flags | BDRV_O_RDWR;
+    flags = bs->open_flags | BDRV_O_CACHE_WB | BDRV_O_RDWR;
 
     /* See if we have a backing HD we can use to create our new image
      * on top of. */
@@ -3510,7 +3525,7 @@ void qmp_drive_mirror(const char *device, const char *target,
         format = mode == NEW_IMAGE_MODE_EXISTING ? NULL : bs->drv->format_name;
     }
 
-    flags = bs->open_flags | BDRV_O_RDWR;
+    flags = bs->open_flags | BDRV_O_CACHE_WB | BDRV_O_RDWR;
     source = backing_bs(bs);
     if (!source && sync == MIRROR_SYNC_MODE_TOP) {
         sync = MIRROR_SYNC_MODE_FULL;
diff --git a/tests/qemu-iotests/142 b/tests/qemu-iotests/142
index 8aa50f8..80834b5 100755
--- a/tests/qemu-iotests/142
+++ b/tests/qemu-iotests/142
@@ -96,36 +96,36 @@ function check_cache_all()
     # bs->backing
 
     echo -e "cache.direct=on on none0"
-    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",cache.direct=on | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",cache.direct=on | grep -e "Cache" -e "[Cc]annot|[Cc]ould not|[Cc]an't"
     echo -e "\ncache.direct=on on file"
-    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",file.cache.direct=on | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",file.cache.direct=on | grep -e "Cache" -e "[Cc]annot|[Cc]ould not|[Cc]an't"
     echo -e "\ncache.direct=on on backing"
-    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.cache.direct=on | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.cache.direct=on | grep -e "Cache" -e "[Cc]annot|[Cc]ould not|[Cc]an't"
     echo -e "\ncache.direct=on on backing-file"
-    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.file.cache.direct=on | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.file.cache.direct=on | grep -e "Cache" -e "[Cc]annot|[Cc]ould not|[Cc]an't"
 
     # cache.writeback is supposed to be inherited by bs->backing; bs->file
     # always gets cache.writeback=on
 
     echo -e "\n\ncache.writeback=off on none0"
-    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",cache.writeback=off | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",cache.writeback=off | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
     echo -e "\ncache.writeback=off on file"
-    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",file.cache.writeback=off | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",file.cache.writeback=off | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
     echo -e "\ncache.writeback=off on backing"
-    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.cache.writeback=off | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.cache.writeback=off | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
     echo -e "\ncache.writeback=off on backing-file"
-    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.file.cache.writeback=off | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.file.cache.writeback=off | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
 
     # cache.no-flush is supposed to be inherited by both bs->file and bs->backing
 
     echo -e "\n\ncache.no-flush=on on none0"
-    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",cache.no-flush=on | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",cache.no-flush=on | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
     echo -e "\ncache.no-flush=on on file"
-    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",file.cache.no-flush=on | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",file.cache.no-flush=on | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
     echo -e "\ncache.no-flush=on on backing"
-    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.cache.no-flush=on | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.cache.no-flush=on | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
     echo -e "\ncache.no-flush=on on backing-file"
-    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.file.cache.no-flush=on | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.file.cache.no-flush=on | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
 }
 
 echo
@@ -218,7 +218,7 @@ info block image
 info block blkdebug
 info block file"
 
-echo "$hmp_cmds" | run_qemu -drive if=none,file="blkdebug::json:{\"filename\":\"$TEST_IMG\",,\"cache\":{\"writeback\":false,,\"direct\":false}}",node-name=image,file.node-name=blkdebug,file.image.node-name=file | grep "Cache"
+echo "$hmp_cmds" | run_qemu -drive if=none,file="blkdebug::json:{\"filename\":\"$TEST_IMG\",,\"cache\":{\"direct\":false}}",node-name=image,file.node-name=blkdebug,file.image.node-name=file | grep "Cache"
 
 echo
 echo "=== Check that referenced BDSes don't inherit ==="
@@ -234,35 +234,35 @@ function check_cache_all_separate()
     # Check cache.direct
 
     echo -e "cache.direct=on on blk"
-    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img",cache.direct=on | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img",cache.direct=on | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
     echo -e "\ncache.direct=on on file"
-    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file",cache.direct=on -drive "$drv_img" | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file",cache.direct=on -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
     echo -e "\ncache.direct=on on backing"
-    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk",cache.direct=on -drive "$drv_file" -drive "$drv_img" | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk",cache.direct=on -drive "$drv_file" -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
     echo -e "\ncache.direct=on on backing-file"
-    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile",cache.direct=on -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img" | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile",cache.direct=on -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
 
     # Check cache.writeback
 
     echo -e "\n\ncache.writeback=off on blk"
-    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img",cache.writeback=off | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img",cache.writeback=off | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
     echo -e "\ncache.writeback=off on file"
-    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file",cache.writeback=off -drive "$drv_img" | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file",cache.writeback=off -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
     echo -e "\ncache.writeback=off on backing"
-    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk",cache.writeback=off -drive "$drv_file" -drive "$drv_img" | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk",cache.writeback=off -drive "$drv_file" -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
     echo -e "\ncache.writeback=off on backing-file"
-    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile",cache.writeback=off -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img" | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile",cache.writeback=off -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
 
     # Check cache.no-flush
 
     echo -e "\n\ncache.no-flush=on on blk"
-    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img",cache.no-flush=on | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img",cache.no-flush=on | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
     echo -e "\ncache.no-flush=on on file"
-    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file",cache.no-flush=on -drive "$drv_img" | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file",cache.no-flush=on -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
     echo -e "\ncache.no-flush=on on backing"
-    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk",cache.no-flush=on -drive "$drv_file" -drive "$drv_img" | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk",cache.no-flush=on -drive "$drv_file" -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
     echo -e "\ncache.no-flush=on on backing-file"
-    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile",cache.no-flush=on -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img" | grep "Cache"
+    echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile",cache.no-flush=on -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
 }
 
 echo
diff --git a/tests/qemu-iotests/142.out b/tests/qemu-iotests/142.out
index abe94c3..5dd5bd0 100644
--- a/tests/qemu-iotests/142.out
+++ b/tests/qemu-iotests/142.out
@@ -66,22 +66,13 @@ cache.writeback=off on none0
     Cache mode:       writeback
 
 cache.writeback=off on file
-    Cache mode:       writeback
-    Cache mode:       writethrough
-    Cache mode:       writeback
-    Cache mode:       writeback
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,file.cache.writeback=off: Can't set writethrough mode except for the root
 
 cache.writeback=off on backing
-    Cache mode:       writeback
-    Cache mode:       writeback
-    Cache mode:       writethrough
-    Cache mode:       writeback
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
 
 cache.writeback=off on backing-file
-    Cache mode:       writeback
-    Cache mode:       writeback
-    Cache mode:       writeback
-    Cache mode:       writethrough
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.file.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
 
 
 cache.no-flush=on on none0
@@ -147,25 +138,13 @@ cache.writeback=off on none0
     Cache mode:       writeback
 
 cache.writeback=off on file
-    Cache mode:       writeback
-    Cache mode:       writeback
-    Cache mode:       writethrough
-    Cache mode:       writeback
-    Cache mode:       writeback
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,file.cache.writeback=off: Can't set writethrough mode except for the root
 
 cache.writeback=off on backing
-    Cache mode:       writeback
-    Cache mode:       writeback
-    Cache mode:       writeback
-    Cache mode:       writethrough
-    Cache mode:       writeback
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
 
 cache.writeback=off on backing-file
-    Cache mode:       writeback
-    Cache mode:       writeback
-    Cache mode:       writeback
-    Cache mode:       writeback
-    Cache mode:       writethrough
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.file.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
 
 
 cache.no-flush=on on none0
@@ -230,22 +209,13 @@ cache.writeback=off on none0
     Cache mode:       writeback, direct
 
 cache.writeback=off on file
-    Cache mode:       writeback, direct
-    Cache mode:       writethrough, direct
-    Cache mode:       writeback, direct
-    Cache mode:       writeback, direct
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,file.cache.writeback=off: Can't set writethrough mode except for the root
 
 cache.writeback=off on backing
-    Cache mode:       writeback, direct
-    Cache mode:       writeback, direct
-    Cache mode:       writethrough, direct
-    Cache mode:       writeback, direct
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
 
 cache.writeback=off on backing-file
-    Cache mode:       writeback, direct
-    Cache mode:       writeback, direct
-    Cache mode:       writeback, direct
-    Cache mode:       writethrough, direct
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.file.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
 
 
 cache.no-flush=on on none0
@@ -306,22 +276,13 @@ cache.writeback=off on none0
     Cache mode:       writeback, direct
 
 cache.writeback=off on file
-    Cache mode:       writeback, direct
-    Cache mode:       writethrough, direct
-    Cache mode:       writeback, direct
-    Cache mode:       writeback, direct
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,file.cache.writeback=off: Can't set writethrough mode except for the root
 
 cache.writeback=off on backing
-    Cache mode:       writeback, direct
-    Cache mode:       writeback, direct
-    Cache mode:       writethrough, direct
-    Cache mode:       writeback, direct
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
 
 cache.writeback=off on backing-file
-    Cache mode:       writeback, direct
-    Cache mode:       writeback, direct
-    Cache mode:       writeback, direct
-    Cache mode:       writethrough, direct
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.file.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
 
 
 cache.no-flush=on on none0
@@ -387,25 +348,13 @@ cache.writeback=off on none0
     Cache mode:       writeback
 
 cache.writeback=off on file
-    Cache mode:       writeback, direct
-    Cache mode:       writeback
-    Cache mode:       writethrough
-    Cache mode:       writeback
-    Cache mode:       writeback
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,file.cache.writeback=off: Can't set writethrough mode except for the root
 
 cache.writeback=off on backing
-    Cache mode:       writeback, direct
-    Cache mode:       writeback
-    Cache mode:       writeback
-    Cache mode:       writethrough
-    Cache mode:       writeback
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
 
 cache.writeback=off on backing-file
-    Cache mode:       writeback, direct
-    Cache mode:       writeback
-    Cache mode:       writeback
-    Cache mode:       writeback
-    Cache mode:       writethrough
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.file.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
 
 
 cache.no-flush=on on none0
@@ -440,7 +389,7 @@ cache.no-flush=on on backing-file
 
     Cache mode:       writethrough, direct, ignore flushes
     Cache mode:       writeback, direct, ignore flushes
-    Cache mode:       writethrough, ignore flushes
+    Cache mode:       writeback, ignore flushes
 
 === Check that referenced BDSes don't inherit ===
 
@@ -722,22 +671,13 @@ cache.writeback=off on none0
     Cache mode:       writeback, direct
 
 cache.writeback=off on file
-    Cache mode:       writeback
-    Cache mode:       writethrough
-    Cache mode:       writeback, direct
-    Cache mode:       writeback, direct
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,file.cache.writeback=off: Can't set writethrough mode except for the root
 
 cache.writeback=off on backing
-    Cache mode:       writeback
-    Cache mode:       writeback
-    Cache mode:       writethrough, direct
-    Cache mode:       writeback, direct
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
 
 cache.writeback=off on backing-file
-    Cache mode:       writeback
-    Cache mode:       writeback
-    Cache mode:       writeback, direct
-    Cache mode:       writethrough, direct
+QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.file.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
 
 
 cache.no-flush=on on none0
-- 
1.8.3.1