Re: [PATCH 2/5] vvfat: move fat_type check prior to size setup

[Kevin Wolf <kwolf@redhat.com>]

Am 29.10.2025 um 14:58 hat BALATON Zoltan geschrieben:
> On Wed, 29 Oct 2025, Clément Chigot wrote:
> > On Thu, Oct 23, 2025 at 8:40 PM Kevin Wolf <kwolf@redhat.com> wrote:
> > > 
> > > Am 03.09.2025 um 09:57 hat Clément Chigot geschrieben:
> > > > This allows to handle the default FAT size in a single place and make the
> > > > following part taking care only about size parameters. It will be later
> > > > moved away in a specific function.
> > > > 
> > > > Setting a floppy disk of 1MB is no longer possible as it was a side
> > > > effect of passing "fat-type=12". To be precise there were three cases:
> > > >  - fat-type undefined (aka default): a fat12 2MB disk
> > > >  - fat-type=16: a fat16 2Mb disk
> > > >  - fat-type=12: a fat12 1Mb disk
> > > 
> > > That's quite a strange interface!
> > > 
> > > If we're touching it anyway, I would change it to make the more common
> > > format (1.44 MB) the default for FAT12 and make the 2.88 MB FAT12 floppy
> > > temporarily unavailable and later require an explicit size. This way
> > > both sizes would still be available using the fat-type.
> > 
> > I'm a bit hesitant to change the default behavior as people might be
> > using it without clear knowledge of it. True, "floppy" is probably not
> > a widely used feature but still.
> > Do QEMU have some specific guidelines when changing such default
> > behavior ? Adding a warning ? Or just a comment in the changelog would
> > be enough ?
> 
> https://www.qemu.org/docs/master/about/deprecated.html
> 
> Usually you'd add a warning and list it in the docs as deprecated then can
> make the change after it was deperecated for two releases.

We're not removing any functionality, just changing the default. So I
don't think the deprecation period applies.

In general, our stance is that we can change defaults whenever we want
to, and if you don't want to be surprised by changing defaults, you need
to specify the option explicitly. What's a bit strange about the vvfat
interface is that the default actually represents a configuration that
can't even be expressed explicitly at the moment.

So it is a special case in a way, but given that this is vvfat, which is
known to be unstable, not widely used outside of the occasional manual
use and not supported by libvirt, I'm willing to just make the change.

Kevin