[Qemu-devel] [BUG?] vvfat vs. pc-1.1 isa-fdc.check_media_rate=off

[Qemu-devel] [BUG?] vvfat vs. pc-1.1 isa-fdc.check_media_rate=off

[Philipp Hahn]

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Hello,

I tried to create a virtual VFAT floppy image on-the-fly with qemi(-kvm)-1.1.1 
using the command line argument
  -drive file=fat:floppy:"$WORK/floppy",if=floppy,index=0

Everything worked fine until I added the additional "-M pc-0.14" command line 
argument: Then I only get an empty floppy image.
I narrowed this down to the PC_COMPAT_1_0 definition in hw/pc_piix.c, which 
added
           .driver   = "isa-fdc",\
            .property = "check_media_rate",\
            .value    = "off",\
to the compat settings. If I also add an
  -global isa-fdc.check_media_rate=on
to explicitly overwrite that setting, vvfat also works with pc-1.0 and 
earlier.

I didn't unsterstand from reading the code what that flag is supposed to do, 
so can somebody give me some help to understand what goes wrong here.

I think vvfat needs to be patched to also work with check_media_rate=off.

Sincerely
Philipp
-- 
Philipp Hahn           Open Source Software Engineer      hahn@univention.de
Univention GmbH        be open.                       fon: +49 421 22 232- 0
Mary-Somerville-Str.1  D-28359 Bremen                 fax: +49 421 22 232-99
                                                   http://www.univention.de/

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Re: [Qemu-devel] [BUG?] vvfat vs. pc-1.1 isa-fdc.check_media_rate=off

[Hervé Poussineau]

Hi,

Philipp Hahn a écrit :
> I tried to create a virtual VFAT floppy image on-the-fly with qemi(-kvm)-1.1.1 
> using the command line argument
>   -drive file=fat:floppy:"$WORK/floppy",if=floppy,index=0

Strange, I just tested today's QEMU git, and I have the opposite problem:
- it works with -M pc-0.14
- it doesn't work with -M pc
- it works with -M pc -global isa-fdc.check_media_rate=off

My test directory only contains one file of 1228800 bytes, and I'm using 
MSDOS as guest OS.

> Everything worked fine until I added the additional "-M pc-0.14" command line 
> argument: Then I only get an empty floppy image.
> I narrowed this down to the PC_COMPAT_1_0 definition in hw/pc_piix.c, which 
> added
>            .driver   = "isa-fdc",\
>             .property = "check_media_rate",\
>             .value    = "off",\
> to the compat settings. If I also add an
>   -global isa-fdc.check_media_rate=on
> to explicitly overwrite that setting, vvfat also works with pc-1.0 and 
> earlier.
 >
> I didn't unsterstand from reading the code what that flag is supposed to do, 
> so can somebody give me some help to understand what goes wrong here.

You can have multiple floppy formats, the most known one is 1.44 MB.
Each floppy format requires a specific media rate transfer to be 
programmed on the floppy controller.

Before 1.1, QEMU was not checking if the current transfer rate was the 
correct one. isa-fdc.check_media_rate parameter enables/disables this 
check; it is on by default on 1.1+, and off by default on previous versions.

vvfat is creating by default 2.88 floppies, which don't have the same 
transfer rate than 1.44 floppies.

Here, you're probably hitting a SeaBIOS bug, which doesn't try multiple 
transfers rates when reading a floppy.

For my case, I can either request floppy controller to not check 
transfer rates (-global isa-fdc.check_media_rate=off), or request vvfat 
to create a 1.44 floppy:
-drive file=fat:12:floppy:"$WORK/floppy",if=floppy,index=0

> I think vvfat needs to be patched to also work with check_media_rate=off.

As said, you may experience a SeaBIOS bug.

Regards,

Hervé