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.
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
