This patchset adds support for the SCSI WRITE SAME with unmap command, which allows reclaiming free space from a backing image.
The user facing implementation is pretty complete, but not really efficient because the underlying bdrv_discard implementation doesn't use the aio implementation yet. The reason for that is that the SCSI layer doesn't really allow any asynchronous commands except for READ/WRITE by design. The only support backend so far is the XFS hole punching ioctl, but others can be added easily when they become available. A virtio implementation for a discard command would also be pretty easy, but until we actually support a better backend then a plain sparse file it's not worth using for production enviroments anyway, but more for playing with the thin provisioning infrastructure, or observing guest behaviour when TRIM / unmap is supported. If the support is enabled and the backend doesn't support hole punching the WRITE SAME command becomes a no-op so that migration from hosts supporting or not supporting it works. Version 4: - incorporate Kevin's review comments for the scsi patch - update to the current block queue - drop ide TRIM support, as it doesn't easily port to the refactoring of the IDE code Version 3: - refactor IDE dma support code - proper brace obsfucation - fix compile without xfs headers - use bool instead of int for a one-byte flag Version 2: - replace tabs with spaces - return -ENOMEDIUM from bdrv_discard if there's no driver assigned - actually list the TP EVPD page as supported when querying for supported EVPD pages