Re: [PATCH] btrfs: add io_uring interface for encoded reads

[David Sterba <dsterba@suse.cz>]

On Mon, Aug 12, 2024 at 08:17:43PM +0100, Pavel Begunkov wrote:
> On 8/12/24 17:58, David Sterba wrote:
> > On Mon, Aug 12, 2024 at 05:10:15PM +0100, Pavel Begunkov wrote:
> >> And the last point, I'm surprised there are two versions of
> >> btrfs_ioctl_encoded_io_args. Maybe, it's a good moment to fix it if
> >> we're creating a new interface.
> >>
> >> E.g. by adding a new structure defined right with u64 and such, use it
> >> in io_uring, and cast to it in the ioctl code when it's x64 (with
> >> a good set of BUILD_BUG_ON sprinkled) and convert structures otherwise?
> > 
> > If you mean the 32bit version of the ioctl struct
> > (btrfs_ioctl_encoded_io_args_32), I don't think we can fix it. It's been
> 
> Right, I meant btrfs_ioctl_encoded_io_args_32. And to clarify, nothing
> can be done for the ioctl(2) part, I only suggested to have a single
> structure when it comes to io_uring.
> 
> > there from the beginning and it's not a mistake. I don't remember the
> > details why and only vaguely remember that I'd asked why we need it.
> > Similar 64/32 struct is in the send ioctl but that was a mistake due to
> > a pointer being passed in the structure and that needs to be handled due
> > to different type width.
> 
> Would be interesting to learn why, maybe Omar remembers? Only two
> fields are not explicitly sized, both could've been just u64.
> The structure iov points to (struct iovec) would've had a compat
> flavour, but that doesn't require a separate
> btrfs_ioctl_encoded_io_args.

Found it:

"why don't we avoid the send 32bit workaround"
https://lore.kernel.org/linux-btrfs/20190828120650.GZ2752@twin.jikos.cz/

"because big-endian"
https://lore.kernel.org/linux-btrfs/20190903171458.GA7452@vader/