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

[Christoph Hellwig <hch@lst.de>]

On Fri, Nov 15, 2024 at 04:40:58PM +0000, Pavel Begunkov wrote:
>> So?  If we have a strong enough requirement for something else we
>> can triviall add another opcode.  Maybe we should just add different
>> opcodes for read/write with metadata so that folks don't freak out
>> about this?
>
> IMHO, PI is not so special to have a special opcode for it unlike
> some more generic read/write with meta / attributes, but that one
> would have same questions.

Well, apparently is one the hand hand not general enough that you
don't want to give it SQE128 space, but you also don't want to give
it an opcode.

Maybe we just need make it uring_cmd to get out of these conflicting
requirements.

Just to make it clear: I'm not a huge fan of a separate opcode or
uring_cmd, but compared to the version in this patch it is much better.

> PI as a special case. And that's more of a problem of the static
> placing from previous version, e.g. it wouldn't be a problem if in the
> long run it becomes sth like:
>
> struct attr attr, *p;
>
> if (flags & META_IN_USE_SQE128)
> 	p = sqe + 1;
> else {
> 	copy_from_user(&attr);
> 	p = &attr;
> }
>
> but that shouldn't be PI specific.

Why would anyone not use the SQE128 version?