This patchset adds support for the ATA TRIM and SCSI WRITE SAME with unmap commands, which allow 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, and implementing the ATA TRIM command with it's multiple ranges is rather painful, and combined with the SCSI limitation I didn't bother yet. The only backend support 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 TRIM / WRITE SAME commands become no-ops so that migration from hosts supporting or not supporting it works.