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[Bug 1608802] Re: READ_DMA (0xC8) command does not work correctly

From: Thomas Huth
Subject: [Bug 1608802] Re: READ_DMA (0xC8) command does not work correctly
Date: Wed, 18 Nov 2020 10:26:34 -0000

The QEMU project is currently considering to move its bug tracking to another 
system. For this we need to know which bugs are still valid and which could be 
closed already. Thus we are setting older bugs to "Incomplete" now.
If you still think this bug report here is valid, then please switch the state 
back to "New" within the next 60 days, otherwise this report will be marked as 
"Expired". Or mark it as "Fix Released" if the problem has been solved with a 
newer version of QEMU already. Thank you and sorry for the inconvenience.

** Changed in: qemu
       Status: Confirmed => Incomplete

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  READ_DMA (0xC8) command does not work correctly

Status in QEMU:

Bug description:
  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:

      Within this zip file is a.img. This is a freeDOS bootable
      floppy.  Emulate it with QEMU and then at the DOS prompt, run

      It will say that the drive is not ready.
       "Drive never became ready"
      (along with a few other lines of text)

      The source for this test suite is also included.
       c8bug.c is the c source code
       c8bug.h is the header file
       ctype.h is a quick way to get 'bit8u' type defines
       timer.h is a delay routine from another project I have
      (The base IO addresses are assumed and set at the top of c8bug.c)
      (compiled with DJGPP for DPMI)

      q.bat is my command line for QEMU

      On all other machines and VirtualBox, the controller is ready
      for me to receive the sector data.
       "Drive is ready to transmit data..."

      However, in QEMU, the controller never becomes ready.
       "Drive never became ready"

  The bug was reported for QEMU for Windows, but I can confirm that QEMU
  for Linux also shows that behaviour, while VirtualBox works.

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