On 2025/03/10 18:10, Peter Krempa wrote:
On Mon, Mar 10, 2025 at 09:05:28 +0000, Daniel P. Berrangé wrote:
On Sat, Mar 08, 2025 at 02:57:41PM +0900, Akihiko Odaki wrote:
usb-storage is a compound device that automatically creates a USB mass
storage device and a SCSI device as its backend. Unfortunately it lacks
some configuration options that are usually present with a SCSI device,
and cannot represent CD-ROM in particular.
Replace usb-storage with usb-bot, which can be combined with a manually
created SCSI device. libvirt will configure the SCSI device in a way
identical with how QEMU does for usb-storage except that now it respects
a configuration option to represent CD-ROM.
Is this change compatible for QEMU's VMState aka migration ABI compatible ?
Based on my tests so far it seems compatible in terms of VMstate and
also guest ABI at least to the first glance (lsusb, lsscsi -v etc) seems
identical at least for the default 'read-write'/non-cdrom disk.
I didn't yet test migrating with a cdrom to an older daemon though,
which based on the above paragraph should have guest-visible difference.
It may work but QEMU doesn't guarantee that. Do you have a suggestion to
deal with migration?
We can keep using usb-storage for hard disk to avoid breaking, but for
CD-ROM, we will need to revive the -drive option support (it was used
instead of -blockdev and able to create a CD-ROM drive with usb-storage).
Honestly I don't think anybody migrates a VM with a USB storage and I
guess it's not worth keeping usb-storage and the -drive.
Regards,
Akihiko Odaki
