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

[Jens Axboe <axboe@kernel.dk>]

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. 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.

-- 
Jens Axboe