Re: [PATCH liburing 1/1] man: add io_uring_register_region.3

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

On 1/14/26 17:23, Jens Axboe wrote:
> On 1/14/26 9:04 AM, Pavel Begunkov wrote:
>> On 1/14/26 14:54, Pavel Begunkov wrote:
>>> On 1/14/26 14:42, Pavel Begunkov wrote:
>>>> On 1/13/26 22:37, Jens Axboe wrote:
>>>>> On 1/13/26 2:31 PM, Gabriel Krisman Bertazi wrote:
>>>>>> Pavel Begunkov <asml.silence@gmail.com> writes:
>>>>>>
>>>>>>> Describe the region API. As it was created for a bunch of ideas in mind,
>>>>>>> it doesn't go into details about wait argument passing, which I assume
>>>>>>> will be a separate page the region description can refer to.
>>>>>>>
>>>>>>
>>>>>> Hey, Pavel.
>>>>>
>>>>> I did a bunch of spelling and phrasing fixups when applying, can you
>>>>> take a look at the repo and send a patch for the others? Thanks!
>>>>
>>>> "Upon successful completion, the memory region may then be used, for
>>>> example, to pass waiting parameters to the io_uring_enter(2) system
>>>> call in a more efficient manner as it avoids copying wait related data
>>>> for each wait event."
>>>>
>>>> Doesn't matter much, but this change is somewhat misleading. Both copy
>>>> args same number of times (i.e. unsafe_get_user() instead of
>>>> copy_from_user()), which is why I was a bit vague with that
>>>> "in an efficient manner".
>>>
>>> Hmm, actually the normal / non-registered way does make an extra
>>> copy, even though it doesn't have to.
>>
>> And the compiler is smart enough to optimise it out since
>> it's all on stack.
> 
> Not sure I follow these emails. For the normal case,
> io_validate_ext_arg() copies in the args via a normal user copy, which
> depending on options and the arch (or even sub-arch, amd more expensive)
> is more or less expensive.

In the end, after prep that is still just a move instruction, e.g.
for x86. And it loads into a register and stores it into ext_arg,
just like with registration. User copy needs to prepare page fault
handling / etc., which could be costly (e.g. I see stac + lfence
in asm), but that's not exactly about avoiding copies.

> For the registered case, it's a simple memory
> dereference. Doesn't cover the signal parts as I believe those are way
> less commonly used.
-- 
Pavel Begunkov