Re: [Qemu-devel] [PULL 075/118] macio: handle non-block ATAPI DMA transfers

[Mark Cave-Ayland]

On 23/06/14 20:26, BALATON Zoltan wrote:

(add Kevin to CC)

> > > I'm afraid as you're the only person that can boot MorphOS this far
> > > then we need you to diagnose and suggest a suitable alternative by
> > > comparing the before and after output. Since MacOS is already a
> > > supported client then if no solution can be found then it is likely
> > > that this patch will be reverted :(
> >
> > So should I revert the patch for now? We're already in soft freeze.

Well let's see if Zoltan can make any headway with debugging over the 
next few days; if there's no progress by the weekend then sadly my 
recommendation would be to revert in time for -rc0 as this definitely 
causes intermittent boot failures in Darwin for me.

> It would be nicer if it could be fixed instead of reverting. You could
> help detangling the macio.c code for a start.

Just to clarify here: the macio/DBDMA code is quite complicated, but 
this is because this device has to work around to the fact that 
currently the DMA I/O routines currently need sector alignment whereas 
macio requires byte-level alignment. There has been quite a lot of work 
at the lower levels to support byte-level alignment (see Kevin's series 
at http://lists.gnu.org/archive/html/qemu-devel/2014-01/msg02163.html) 
but until we can specify transfers to byte granularity in the 
dma_bdrv_*() APIs then there isn't much we can do to clean up the 
macio.c code.

Kevin, are there any plans to bubble the byte-granularity block layer 
changes up to the dma_bdrv_*() APIs in the near future?

Please bear in mind that QEMU supports a large number of OSs, and there 
is already an enthusiastic group of people using Alex's OS X work (see 
emaculation for many examples) so introducing an intermittent fault on a 
supported OS is not an option here.

I should also re-emphasise that Alex/Andreas work on many different 
parts of QEMU, and my work is currently unsponsored so while we are all 
keen to improve QEMU to the point where it can emulate new OSs such as 
MorphOS, it's not the case that we can simply drop what we are doing at 
the time to focus on an issue that affects a single OS which is new and 
currently unsupported.

Now I think it's fair to say that I've spent quite a few hours helping 
you and coming up with the original version of this patch, and I'm glad 
that you are now seeing success with this. But what is important to us 
right now heading towards a release is that patches don't cause any 
regressions.

All I can say is that debugging this stuff isn't easy, particularly with 
MorphOS which has some rather unusual behaviours. But what we really 
need from you now over the next few days is for you to compare the debug 
output between the working and non-working cases and figure out if we 
can fix this in time for the 2.1 release. You have everything you need 
(including my acceptance test of booting both MorphOS and Darwin ISOs), 
so time to take a deep breath and begin what should be a challenging yet 
ultimately rewarding debugging process :)


ATB,

Mark.