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

[Kanchan Joshi]

On 9/25/2024 11:27 AM, Hannes Reinecke wrote:
>> @@ -98,6 +98,11 @@ struct io_uring_sqe {
>>               __u64    addr3;
>>               __u64    __pad2[1];
>>           };
>> +        struct {
>> +            /* To send per-io hint type/value with write command */
>> +            __u64    hint_val;
>> +            __u8    hint_type;
>> +        };
> Why is 'hint_val' 64 bits? Everything else is 8 bytes, so wouldn't it
> be better to shorten that? 

Right, within kernel hint is stored as 8bits value.
But I chose not because how kernel stores hint internally (which may 
change at any time) but how the existing F_SET_RW_HINT interface exposed 
this to user space. It expects u64.

If we do 8bits interface here, application needs to learn that for the 
same lifetime hint it needs u64 for fcntl interface, but u8 for io_uring 
interface. That seems a bit confusing.

Also, in future if we do support another hint type, we may be able to 
pass hint_val beyond what can be supported by u8.

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).

[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 */
};