Re: [Qemu-devel] [PULL 48/49] i386: populate floppy drive information in DSDT

[Roman Kagan]

On Wed, Feb 10, 2016 at 11:14:30AM -0500, John Snow wrote:
> On 02/09/2016 01:48 PM, Michael S. Tsirkin wrote:
> > On Tue, Feb 09, 2016 at 07:36:12PM +0100, Laszlo Ersek wrote:
> >> Implementing this in QEMU would require:
> >> - inventing virt-only registers for the FDC that provide the current
> >> disk geometry,
> 
> We do secretly have these registers, but of course there's no spec to
> interpreting what they mean. For instance, what do they read when the
> drive is empty? I am not sure that information is codified in the ACPI spec.

There are various possibilities, like returning False for the
corresponding drive in _FDE (Floppy Disk Enumerate) object, or returning
0 aka unknown as drive type in _FDI.  Anyway I'd hate needing to expose
all of that in an ACPI-accessible form, this is going to become too fat.

> Could be wrong, you seemed to indicate that the ACPI spec said that the
> info matches what you get from a certain legacy bios routine, but I
> don't know the specifics of that routine, either.

Indeed, it's supposed to do the same as
https://en.wikipedia.org/wiki/INT_13H#INT_13h_AH.3D08h:_Read_Drive_Parameters

and, as I wrote in another mail, the SeaBIOS implementation here is
rather simplistic.

> Roman, does the 0xFF "empty disk geometry" hack appear to work for
> Windows 10?
> 
> Maybe it's fine enough as-is, as per Laszlo's good synopsis here.

I'll test and let you know.

Roman.