Re: [PATCH v6 3/3] io_uring: enable per-io hinting capability

[Pavel Begunkov <asml.silence@gmail.com>]

On 9/25/24 12:09, Kanchan Joshi wrote:
> On 9/25/2024 11:27 AM, Hannes Reinecke wrote:
...
> As it stands the new struct will introduce
>> a hole of 24 bytes after 'hint_type'.
> 
> This gets implicitly padded at this point [1][2], and overall size is
> still capped by largest struct (which is of 16 bytes, placed just above
> this).

For me it's about having hardly usable in the future by anyone else
7 bytes of space or how much that will be. Try to add another field
using those bytes and endianess will start messing with you. And 7
bytes is not that convenient.

I have same problem with how commands were merged while I was not
looking. There was no explicit padding, and it split u64 into u32
and implicit padding, so no apps can use the space to put a pointer
anymore while there was a much better option of using one of existing
4B fields.


> [1] On 64bit
> »       union {
> »       »       struct {
> »       »       »       __u64      addr3;                /*    48     8 */
> »       »       »       __u64      __pad2[1];            /*    56     8 */
> »       »       };                                       /*    48    16 */
> »       »       struct {
> »       »       »       __u64      hint_val;             /*    48     8 */
> »       »       »       __u8       hint_type;            /*    56     1 */
> »       »       };                                       /*    48    16 */
> »       »       __u64              optval;               /*    48     8 */
> »       »       __u8               cmd[0];               /*    48     0 */
> »       };                                               /*    48    16 */
> 
> »       /* size: 64, cachelines: 1, members: 13 */
> 
> [2] On 32bit
> 
> »       union {
> »       »       struct {
> »       »       »       __u64      addr3;                /*    48     8 */
> »       »       »       __u64      __pad2[1];            /*    56     8 */
> »       »       };                                       /*    48    16 */
> »       »       struct {
> »       »       »       __u64      hint_val;             /*    48     8 */
> »       »       »       __u8       hint_type;            /*    56     1 */
> »       »       };                                       /*    48    12 */
> »       »       __u64              optval;               /*    48     8 */
> »       »       __u8               cmd[0];               /*    48     0 */
> »       };                                               /*    48    16 */
> 
> »       /* size: 64, cachelines: 1, members: 13 */
> };

-- 
Pavel Begunkov