[Qemu-devel] Qemu floppy emulation problems - partially solved

[Qemu-devel] Qemu floppy emulation problems - partially solved

[Johannes Martin]

Hi,

a little while ago we had some discussion about floppy emulation not
working in qemu once an OS has been installed. Our assumption was that
floppy access worked through int13 bios emulation but not really down at
the hardware level.

Today I noticed in Win98 that Windows would only offer to format a floppy
at 180kB. I then booted Win98 with a floppy inserted at boot time and
noticed two things:
- rather than two 'removable disk drives' Win98 now only reports one
  3 1/2" floppy drive (correctly)
- I can actually access that floppy and other floppies I insert later
  (no disk change problem as with CDs)

Observations on other OSes:
- Win95 reports a VFAT error and pretty much dies
- OS/2 can access the floppy that was inserted originally, as soon as I
  try to access any other floppy, qemu dies with a segmentation fault.

Tested on Debian/GNU Linux with a two week old qemu/CVS.

So I guess our initial assumption was at least partially incorrect, the
problem is not (primarily) the fdc hardware emulation in general but the
part OSes need to detect the kind of floppy drive installed. Apparently,
if no virtual floppy is inserted, the detection fails. I haven't actually
looked at the code yet - I imagine it to be quite easy to fix though.

Any hints how to find out why/where qemu segfaults under OS/2? How can I
get a core dump out of qemu? So far, my attempts to debug qemu using gdb
have failed miserably. :(

Thanks
	Johannes

Re: [Qemu-devel] Qemu floppy emulation problems - partially solved

[Johannes Martin]

Hi,

On Tue, 17 Aug 2004, Johannes Martin wrote:
> Today I noticed in Win98 that Windows would only offer to format a floppy
> at 180kB. I then booted Win98 with a floppy inserted at boot time and
> noticed two things:
> - rather than two 'removable disk drives' Win98 now only reports one
>   3 1/2" floppy drive (correctly)
> - I can actually access that floppy and other floppies I insert later
>   (no disk change problem as with CDs)

update:
when I 'eject fda' in the Win98 guest, and try to access the floppy, I
receive hundreds/thousands of
	dma: DMA transfer is not read
and the guest pretty much stalls. 'change fda /dev/zero' gets things
straight again.

Actually, when I start qemu with -fda /dev/zero, the floppy drive is
detected ok. So that would be a very quick fix to the problem.

The only remaining problem is that I get a segfault when I try to format
the virtual floppy when it points to /dev/zero (I would expect a 'disk is
write protected' error).

	Johannes

Re: [Qemu-devel] Qemu floppy emulation problems - partially solved

[malc]

On Tue, 17 Aug 2004, Johannes Martin wrote:

> Hi,
>
> On Tue, 17 Aug 2004, Johannes Martin wrote:
> > Today I noticed in Win98 that Windows would only offer to format a floppy
> > at 180kB. I then booted Win98 with a floppy inserted at boot time and
> > noticed two things:
> > - rather than two 'removable disk drives' Win98 now only reports one
> >   3 1/2" floppy drive (correctly)
> > - I can actually access that floppy and other floppies I insert later
> >   (no disk change problem as with CDs)
>
> update:
> when I 'eject fda' in the Win98 guest, and try to access the floppy, I
> receive hundreds/thousands of
> 	dma: DMA transfer is not read
> and the guest pretty much stalls. 'change fda /dev/zero' gets things
> straight again.

I have changed DMA API with 7th audio patch and by extension had to modify
fdc.c to use it (Jocelyn didn't return calls when i tried to notify him).

So:
  a. I'd suggest using vanilla QEMU when working on fdc for the time being
  b.             getting 7b_aqemu (or later) that tries to keep DMA silent

-- 
mailto:malc@pulsesoft.com