On 22/05/15 19:20, John Snow wrote: > As a post-script, can Darwin/PPC use a different mechanism for ATA at > all, or is macio the sole ATA interface we support here? > > I want to see if I can pinpoint when a "good" bath and when a bad path > diverges with respect to the disk contents ... > > Or if you have other ideas on how to identify the transfer that causes > the issue, I'm all ears.
After a fairly involved weekend, I've managed to track this one down. It was an actually an issue with the chaining of write requests with unaligned accesses, whereby alternating patterns of head, head/tail and tail would end up overwriting the common remainder section and so copy the wrong sector back to the disk. The bad news is that this has definitely been broken since QEMU 2.1+ so anyone using a Darwin/OS X PPC image is very likely to have an installation that won't boot, or corruption in an image if it was originally installed on an older QEMU version :( Note that as I figured out what was happening my code was morphing into a very similar pattern to that of bdrv_co_do_preadv()/bdrv_co_do_pwritev() (except mine still calculated with sectors) and so I bit the bullet and moved them over to match the existing style. This has the advantage that the algorithm is similar to an existing one, plus if unaligned block APIs become available in the future then switching them over should be fairly trivial. I'll post an updated patchset shortly. John, if you could confirm it fixes the issue on your side that would be great. ATB, Mark.
