Re: [Qemu-devel] How to correctly use more than 2 floppy drives?

[John Snow <jsnow@redhat.com>]

On 4/8/19 1:38 AM, Markus Armbruster wrote:
> Hervé Poussineau <hpoussin@reactos.org> writes:
> 
>> Le 05/04/2019 à 12:29, Philippe Mathieu-Daudé a écrit :
>>> Hi,
>>>
>>> I am trying to understand the possible values for the MAX_FD variable
>>> used by the floppy controller model (hw/block/fdc.c).
>>>
>>> Looking at git history:
>>>
>>> - 2004-01-05 7138fcfbf7dd + 8977f3c107ef (Jocelyn Mayer):
>>>    FDC introduced with "#define MAX_FD 2"
>>>
>>> - 2008-04-29 78ae820cfeb0 (Hervé Poussineau):
>>>    Supports up to 4 floppy drives if MAX_FD is set to 4
>>>    Migration stream knows about runtime value of MAX_FD
>>>
>>> - 2009-09-10 d7a6c2703577 (Juan Quintela):
>>>    FDC vmstate-ified
>>>    Migration stream use compile time value of MAX_FD
>>>
>>> Since 7138fcfbf7dd MAX_FD has always been defined as 2.
>>>
>>> Since d7a6c2703577 MAX_FD can not be different than 2 without breaking
>>> migration.
>>>
>>> If I understand correctly migration, first we should change in
>>> vmstate_fdc the user-definable MAX_FD by a constant 2 value.
>>>
>>> Then to be able to use >2 floppy disks I have to modify the the
>>> vmstate.version_id, and
>>>
>>> 1/ add a new field in the vmstate_fdc containing the number of drives
>>> and add code to check >2 and adapt
>>>
>>> or
>>>
>>> 2/ change MAX_FD to 4 for all the codebase, adding some code to migrate
>>> to older FDC with only 2 disks...
>>>
>>> Another option I don't like is:
>>>
>>> 3/ get ride of MAX_FD != 2 and clean the codebase...
>>>
>>> $ git grep '#if MAX_FD'
>>> hw/block/fdc.c:744:#if MAX_FD == 4
>>> hw/block/fdc.c:758:#if MAX_FD == 4
>>> hw/block/fdc.c:1317:#if MAX_FD == 4
>>> hw/block/fdc.c:1340:#if MAX_FD == 4
>>> hw/block/fdc.c:2041:#if MAX_FD == 4
>>> hw/block/fdc.c:2079:#if MAX_FD == 4
>>> hw/block/fdc.c:2104:#if MAX_FD == 4
>>>
>>> Hervé, what board are/were you using with 4 floppy drives?
>>
>> That was only an attempt to support 4 drives in code, as controller was able to do it.
>> However, no emulated board took advantage of it, so that's why it kind-of regressed.
>>
>> Feel free to choose the solution you prefer.
> 
> Since no user has appeared since 2008, it feels safe enough to assume
> that none will appear going forward.  I lean towards your "3/ get ride
> of MAX_FD != 2 and clean the codebase..."  Explain in a comment that we
> emulate only a 2-drive floppy controller, not the genuine IBM PC 4-drive
> floppy controller.
> 

I think I'd rather have MAX_FD set to 2 and a cleaner codebase than a
half-working implementation for 4.

Or, does it actually work with four? I think if Hervé wants to preserve
this feature it should be formalized as a device property and made to
work with migration ... or I am content to remove it.

--js