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

[Michael S. Tsirkin]

On Tue, Feb 09, 2016 at 07:36:12PM +0100, Laszlo Ersek wrote:
> On 02/09/16 17:22, John Snow wrote:
> > 
> > 
> > On 02/09/2016 10:52 AM, Roman Kagan wrote:
> >> On Mon, Feb 08, 2016 at 03:20:47PM -0500, John Snow wrote:
> >>> On 02/08/2016 08:14 AM, Roman Kagan wrote:
> >>>> On Fri, Feb 05, 2016 at 07:25:07PM +0100, Igor Mammedov wrote:
> >>>>>> +    aml_append(fdi,
> >>>>>> +        aml_int(cylinders - 1));  /* Maximum Cylinder Number */
> >>>>> this puts uint64_t(-1) in AML i.e. cylinders == 0 and overflow happens 
> >>>>> here
> >>>>>
> >>>>> CCing Jon
> >>>>
> >>>> I guess this is the effect of John's fdc rework.  I used to think zero
> >>>> geometry was impossible at the time this patch was developed.
> >>>>
> >>>> I wonder if it hasn't been fixed already by
> >>>>
> >>>>   commit fd9bdbd3459e5b9d51534f0747049bc5b6145e07
> >>>>   Author: John Snow <address@hidden>
> >>>>   Date:   Wed Feb 3 11:28:55 2016 -0500
> >>>>
> >>>>       fdc: fix detection under Linux
> >>>
> >>> Yes, hopefully solved on my end. The geometry values for an empty disk
> >>> are not well defined (they certainly don't have any *meaning*) so if you
> >>> are populating tables based on an empty drive, I just hope you also have
> >>> the mechanisms needed to update said tables when the media changes.
> >>
> >> I don't.  At the time the patch was developed there basically were no
> >> mechanisms to update the geometry at all (and this was what you patchset
> >> addressed, in particular, wasn't it?) so I didn't care.
> >>
> > 
> > That's not true.
> > 
> > You could swap different 1.44MB-class diskettes for other geometries,
> > check this out:
> > 
> > static const FDFormat fd_formats[] = {
> >     /* First entry is default format */
> >     /* 1.44 MB 3"1/2 floppy disks */
> >     { FDRIVE_DRV_144, 18, 80, 1, FDRIVE_RATE_500K, },
> >     { FDRIVE_DRV_144, 20, 80, 1, FDRIVE_RATE_500K, },
> >     { FDRIVE_DRV_144, 21, 80, 1, FDRIVE_RATE_500K, },
> >     { FDRIVE_DRV_144, 21, 82, 1, FDRIVE_RATE_500K, },
> >     { FDRIVE_DRV_144, 21, 83, 1, FDRIVE_RATE_500K, },
> >     { FDRIVE_DRV_144, 22, 80, 1, FDRIVE_RATE_500K, },
> >     { FDRIVE_DRV_144, 23, 80, 1, FDRIVE_RATE_500K, },
> >     { FDRIVE_DRV_144, 24, 80, 1, FDRIVE_RATE_500K, },
> > ...
> > 
> > You absolutely could get different sector and track counts before my
> > patchset.
> > 
> > 
> >> Now if it actually has to be fully dynamic it's gonna be more
> >> involved...
> >>
> >>> What do the guests use these values for? Are they fixed at boot?
> >>
> >> Only Windows guests use it so it's hard to tell.  I can only claim that
> >> if I stick bogus values into that ACPI object the guest fails to read
> >> the floppy.
> 
> We discussed this with John a bit on IRC.
> 
> In my opinion, the real mess in this case is in the ACPI spec itself. If
> you re-read the _FDI control method's description, the Package that it
> returns contains *dynamic* geometry data, about the *disk* (not *drive*):
> 
> - Maximum Cylinder Number // Integer (WORD)
> - Maximum Sector Number   // Integer (WORD)
> - Maximum Head Number     // Integer (WORD)
> 
> What this seems to require is: the firmware developer should write ACPI
> code that
> - dynamically accesses the floppy drive / controller (using MMIO or IO
>   port registers),
> - retrieves the geometry of the disk actually inserted,
> - and returns the data nicely packaged.
> 
> In effect, an ACPI-level driver for the disk.
> 
> Now, John explained to me (and please keep in mind that this is my
> personal account of his explanation, not a factual rendering thereof),
> that there used to be no *standard* way to interrogate the current
> disk's geometry, other than trial and error with seeking.
> 
> Apparently in UEFI Windows, Microsoft didn't want to implement this
> trial and error seeking, so -- given there was also no portable
> *hardware spec* to retrieve the same data, directly from the disk drive
> or controller -- they punted the entire question to ACPI. That is, to
> the firmware implementor.
> 
> This is entirely bogus. For one, it ties the platform firmware (the UEFI
> binary in the flash chip on your motherboard) to your specific floppy
> drive / controller. Say good-bye to any independently bought & installed
> floppy drives.
> 
> In theory, a floppy controller that comes with a separate PCI card could
> offer an option ROM with a UEFI driver in it, and that UEFI driver could
> install a separate SSDT with the hardware-matching _FDI in it. Still an
> unreasonable requirement, given that the *only* way Windows can be
> installed unattended (with external device drivers) is to provide those
> drivers on a floppy. Because, end-to-end,
> 
>   do you want unattended UEFI installation with 3rd party drivers?
> 
> translates to
> 
>   better have a PCI-based floppy controller such that its oprom
>   installs an _FDI with dynamic hardware access,
>   *or* have your platform firmware match your floppy hardware
> 
> Implementing this in QEMU would require:
> - inventing virt-only registers for the FDC that provide the current
> disk geometry,
> - and generating AML code that reads those registers
> 
> *or*
> 
> - implementing the trial and error seeking in AML
> 
> Waste of time, don't do it. Microsoft have never documented their usage
> of _FDI. (Their forums are full of confused users whose physical floppy
> drives don't work under UEFI Windows!)
> 
> I'm quite sure the _FDI addition in the ACPI spec is actually from
> Microsoft as well, but of course the *reasoning* / background for _FDI
> is also not public. Microsoft seem to push stuff into the ACPI spec that
> serves them, while conveniently ignoring non-optional parts of the spec
> that they don't feel like supporting (I'm looking at you,
> DataTableRegion). And their level of support is not public.
> 
> So, the last paragraph of Roman's email
> <http://thread.gmane.org/gmane.comp.emulators.qemu.block/7978/focus=8081> 
> remains
> relevant -- do whatever ugly static hack is necessary in QEMU's AML
> generator to restore the one use case to working state that matters:
> unattended installation to a virtio disk.
> 
> Noone in their right mind uses floppy in the guest interactively, and
> even for the unattended installation, floppy is used only because
> Windows is too stupid to work off a CD-ROM fully automatically. (Where
> everything one needs would be interrogable from on hardware / ATAPI / SCSI.)
> 
> IMHO, do the *absolute minimum* to adapt this AML generation patch to
> John's FDC rework, and ignore all dynamic aspects (like media change).
> 
> Thanks
> Laszlo

I'm fine with reporting some static values that make windows work fine.
What I think is wrong is reporting geometry that happens to
match current windows.

-- 
MST