What were you trying to do that didn't work?

While implementig the required changes for RHEL-7343, where the main idea is to remove the unneeded raw driver blockdevs with no additional config on top of protocol driver nodes from the block graph , I've noticed that if a block job will be updating a backing file of a image to point to the raw driver, a protocol name will be recorded into the backing file format field.

Unfortunately libvirt-9.9 and older would not handle that properly and in cases where the protocol is remote images would become unusable.

To address this we need a way to force qemu to write an actual format name into the backing file format field instead of the protocol name.

Please provide the package NVR for which bug is seen:

All current qemu versions. Tested on git v8.2.0-rc1

How reproducible:

Always

Steps to reproduce

(note that I was using libvirt with patches removing the raw driver which are currently not posted yet, I can clarify the steps if needed)

1. Create a backing chain of 3 images, tail of chain is 'raw'
2. Start qemu with the chain, omitting a raw format driver for the 'raw' image
3. do a 'block-commit' from the middle image into the 'raw' tail image.
4. observe file in the backing store format field

Expected results

raw will be recorded into backing store format

Actual results

file/host_device/etc.. is recorded

Patches:

v1: https://lists.gnu.org/archive/html/qemu-block/2023-11/msg00539.html
v2: https://lists.gnu.org/archive/html/qemu-block/2023-11/msg00634.html