[PATCH v9 06/11] io_uring: introduce attributes for read/write and PI support

[Christoph Hellwig]

On Fri, Nov 15, 2024 at 07:03:28PM +0000, Pavel Begunkov wrote:
>>> but that shouldn't be PI specific.
>>
>> Why would anyone not use the SQE128 version?
>
> !SQE128 with user pointer can easily be faster depending on the
> ratio of requests that use SQE128 and don't. E.g. one PI read
> following with a 100 of send/recv on average. copy_from_user
> is not _that_ expensive and we're talking about zeroing an
> extra never used afterwards cache line.

Why would you use the same ring for it?  Remember PI is typically
used by thing like databases.  Everything that does disk I/O
will use it, so optimizing for it actually being used absolutely
makes sense.