Re: [Qemu-block] [Qemu-devel] [Bug 1608802] [NEW] READ_DMA (0xC8) command does not work correctly

[Benjamin David Lunt]

----- Original Message ----- 
> Am 02.08.2016 um 08:52 hat Stefan Weil geschrieben:
> > Am 02.08.2016 um 08:11 schrieb Stefan Weil:
> > > Public bug reported:
> > >
> > > The QEMU PC emulation of DMA does not behave like real hardware or other
> > > virtualization software.
> > >
> > > >From the original bug report (Benjamin David Lunt):
> > >
> > >     Back to the READ_DMA command, it is my conclusion that the
> > >     READ_DMA command, more precisely, the BUS Master part of QEMU is
> > >     in error.  The tests that people have done to see if it works, is
> > >     probably the guest finding out that DMA doesn't work and defaulting
> > >     to PIO, but since the read was successful visually to the user, the
> > >     user assumed the READ_DMA command works, where the guest actually
> > >     defaulted back to PIO transfers without notice.
> > >
> > >     My code works on real hardware (numerous machines), Bochs, and Oracle's
> > >     Virtual Box.
> > >
> > >     ...
> > >
> > >     I have a small test suite, zipped and included at:
> > >     www.fysnet.net/temp/c8bug.zip
> > >
> > >     Within this zip file is a.img. This is a freeDOS bootable
> > >     floppy.  Emulate it with QEMU and then at the DOS prompt, run
> > >     c8bug.exe.
> >
> > Hi John,
> >
> > I got this bug report only recently from a Windows user,
> > but it also occurs on Linux.
> >
> > As I don't know whether this is a regression or whether
> > it is relevant for QEMU 2.7, it would be good if you and
> > maybe more people could have a look on that problem,
> > too.
>
> I don't think it's a regression. I commented with more detail in the bug
> report, and despite the test case being buggy it seems to be true that
> qemu doesn't get the flags completely right (we should set either BSY or
> DRQ, but we do set both). Apparently none of the common drivers check
> for this, though, so it never made any difference.
>
> Kevin

I agree that the controller is either BSY && !DRQ or !BSY && DRQ.
This is explained in section 9.7 of ATAPI v6, Page 354 (doc page 340).

My test was to simply show that the status remains 0xD8.
BSY *and* DRQ always set.

If a patch is made to make QEMU either BSY && !DRQ or !BSY && DRQ
after the DMA command, then I believe it will be correct.

I would guess that the common drivers, as you put it, don't check
for the status at all, but simply wait to see if an interrupt fires.

Thank you,
Ben