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

[Jens Axboe <axboe@kernel.dk>]

On 1/15/26 6:06 AM, Pavel Begunkov wrote:
> On 1/14/26 23:53, Jens Axboe wrote:
> ...
>>>>> 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.
>>
>> Those are implementation details. The user copy is stac/clac, and then
>> the loads. This is what makes it more expensive. I don't want to be
>> writing about stac/clac in the man page, that's irrelevant to the user.
> 
> Confused why would you be thinking about putting that into the
> man page. I'm saying that it claims copy avoidance, but there is
> no difference in the number of copies. It's also uncomfortable
> that it's in a commit with my name attached, while the change
> wouldn't fall under the "language edits" note.

Sheesh let's turn down the sensitivity. If you want it changed, send a
patch. I'm trying to phrase it in such a way that it makes sense to
people without getting into too much detail. It avoids copying from
USERSPACE, which is the expensive part.

I think we've spent enough time on this detail at this point, no?

-- 
Jens Axboe