Re: [PATCH v2 5/5] vvfat: add support for "fat-size" options

[Kevin Wolf <kwolf@redhat.com>]

Am 10.11.2025 um 14:42 hat Markus Armbruster geschrieben:
> Clément Chigot <chigot@adacore.com> writes:
> 
> > On Mon, Nov 10, 2025 at 2:09 PM Markus Armbruster <armbru@redhat.com> wrote:
> >>
> >> Clément Chigot <chigot@adacore.com> writes:
> >>
> >> > On Mon, Nov 10, 2025 at 11:13 AM Markus Armbruster <armbru@redhat.com> wrote:
> >> >>
> >> >> Clément Chigot <chigot@adacore.com> writes:
> >> >>
> >> >> > This allows more flexibility to vvfat backend. The values of "Number of
> >> >> > Heads" and "Sectors per track" are based on SD specifications Part 2.
> >> >> >
> >> >> > Due to the FAT architecture, not all sizes are reachable. Therefore, it
> >> >> > could be round up to the closest available size.
> >> >> >
> >> >> > FAT32 has not been adjusted and thus still default to 504 Mib.
> >> >> >
> >> >> > For floppy, only 1440 Kib and 2880 Kib are supported.
> >> >> >
> >> >> > Signed-off-by: Clément Chigot <chigot@adacore.com>
> >> >>
> >> >> [...]
> >> >>
> >> >> > diff --git a/qapi/block-core.json b/qapi/block-core.json
> >> >> > index 8a479ba090..0bcb360320 100644
> >> >> > --- a/qapi/block-core.json
> >> >> > +++ b/qapi/block-core.json
> >> >> > @@ -3478,11 +3478,17 @@
> >> >> >  #     (default: true)
> >> >> >  #     (since 10.2)
> >> >> >  #
> >> >> > +# @fat-size: size of the device in bytes.  Due to FAT underlying
> >> >> > +#     architecture, this size can be rounded up to the closest valid
> >> >> > +#     size.
> >> >> > +#     (since 10.2)
> >> >> > +#
> >> >>
> >> >> Can you explain again why you moved from @size to @fat-size?
> >> >
> >> > Just to be sure, you mean in the above comment, in the commit message or both ?
> >>
> >> Just to me, because I'm not sure I like the change, but that may well be
> >> due to a lack of understanding of your reasons.
> >
> > Naming `fat-size` instead of `size` ensures the parameter is only
> > recognized by the vvfat backend. In particular, it will be refused by
> > the default raw format, avoiding confusion:
> >  "-drive file=fat:<path>,size=256M" results in a 504M FAT disk
> > truncated to 256M, raw format being implicit.
> >  "-drive file=fat:<path>,fat-size=256M" is refused. "fat-size" is
> > unsupported by raw format.
> 
> I figure throwing in format=raw to make raw format explicit doesn't
> change anything.  Correct?
> 
> >  "-drive file=fat:<path>,format=vvfat,fat-size=256M" results in a 256M FAT disk.
> >  "-drive file=fat:<path>,format=vvfat,size=256M" is refused. "size" is
> > unsupported by vvfat format.
> 
> If it was called @size, what behavior would we get?  Just two cases, I
> think:
> 
> 1. With raw format:
> 
>     -drive file=fat:<path>,size=256M

You'd silently get a 504 MiB filesystem truncated to 256 MiB (i.e. a
corrupted file system). It's quite easy to forget format=vvfat, so
something that initially looks like it might be working is not a great
result for this user error.

> 2. Without raw format:
> 
>     -drive file=fat:<path>,format=vvfat,size=256M

This does the thing that you actually want, a 256 MiB file system.

I suggested to rename the vvfat option in v1 to make accidents at least
a bit less likely. I'm not completely sure if "fat-size" is the best
name, though, as it sounds as if it referred to the FAT itself instead
of the FAT filesystem. Maybe "fs-size"?

Kevin