On Wed, Jun 11, 2025 at 12:23:00PM +0200, Christian Brauner wrote:
> > +struct logical_block_metadata_cap {
> > + __u32 lbmd_flags;
> > + __u16 lbmd_interval;
> > + __u8 lbmd_size;
> > + __u8 lbmd_opaque_size;
> > + __u8 lbmd_opaque_offset;
> > + __u8 lbmd_pi_size;
> > + __u8 lbmd_pi_offset;
> > + __u8 lbmd_guard_tag_type;
> > + __u8 lbmd_app_tag_size;
> > + __u8 lbmd_ref_tag_size;
> > + __u8 lbmd_storage_tag_size;
> > + __u8 lbmd_rsvd[17];
>
> Don't do this hard-coded form of extensiblity. ioctl()s are inherently
> extensible because they encode the size. Instead of switching on the
> full ioctl, switch on the ioctl number. See for example fs/pidfs:
Umm, yes and no. The size encoding in the ioctl is great. But having
a few fields in a structure that already has flags allows for much
easier extensions for small amounts of data. Without the reserved
fields, this structure is 15 bytes long. So we'll need at least 1
do pad to a natural alignment. I think adding another 16 (aka
two u64s) seems pretty reasonable for painless extensions.
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