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Date: Wed, 13 Jan 2016 18:49:45 +0300
From: Roman Kagan <rkagan@virtuozzo.com>
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Subject: Re: [Qemu-devel] [PATCH v5 0/6] i386: expose floppy-related objects
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To: Laszlo Ersek <lersek@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Eduardo Habkost <ehabkost@redhat.com>, qemu-block@nongnu.org, "Michael S. Tsirkin" <mst@redhat.com>, qemu-stable@nongnu.org, qemu-devel@nongnu.org, Igor Mammedov <imammedo@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, John Snow <jsnow@redhat.com>, "Dr. David Alan
	Gilbert (git)" <dgilbert@redhat.com>, Richard Henderson <rth@twiddle.net>

On Wed, Jan 13, 2016 at 03:36:18PM +0100, Laszlo Ersek wrote:
> On 12/30/15 21:11, Roman Kagan wrote:
> > Windows on UEFI systems is only capable of detecting the presence and
> > the type of floppy drives via corresponding ACPI objects.
> 
> I'm late to the party, but please allow me a question:
> 
> how did you figure out that UEFI Windows requires this?
> 
> In general, what the ACPI specification says is at best a "guideline"
> for Windows. So how did you prove this was a requirement for Windows?

Well, my statement above that Windows on UEFI can detect floppies *only*
via ACPI is probably a bit stronger than I can actually prove but

- Windows on OVMF didn't see floppies before the patch, while Linux did
  (by querying CMOS)

- a number of sources on the internet hinted that Windows needed ACPI
  assistance for that, e.g.:

  https://www.reactos.org/wiki/UEFI#Floppy
  https://social.technet.microsoft.com/Forums/windows/en-US/f17db175-d146-4518-b2e9-c12a15031222/legacy-floppy-compatibility-with-uefi-boot?forum=w7itprohardware
  https://social.technet.microsoft.com/Forums/windows/en-US/e91ec27b-0c2d-44a3-b949-e77fa810a4c0/windows-7-uefi-fdd-how-to?forum=w7itprohardware

- the links mentioned the need in _FDE object but indicated it only
  allowed for successful enumeration of floppies, not the actual access;
  I proved that experimentally

- the ACPI spec stated that _FDE went in concert with _FDI so I tried it
  and it worked out

Voila.  Besides, I later discovered that a similar research had been
carried out for Parallels proprietary hypervisor, with a similar
outcome.

Roman.