Re: [PATCH] Add ifunc memcpy and memmove for aarch64

[Szabolcs Nagy <szabolcs dot nagy at arm dot com>]

On 09/02/17 11:04, Siddhesh Poyarekar wrote:
> On Thursday 09 February 2017 04:20 PM, Szabolcs Nagy wrote:
>> this is a trap into the kernel at every process startup
>>
>> since this is called very early (dynamic linking case
>> above, static linking case below) i wonder if there
>> could be a way for the user to request midr_el1==0
>> unconditionally (avoiding the overhead and making
>> sure the most generic implementation is used)
> 
> Well you could use tunables to avoid it, but then if a single trap is a
> problem for you then the tunables infra is going to be just as expensive.
> 
> Why is a single trap at startup such a concern though?

ok, it is probably not worth worrying about
(should be around 0.1% of the minimal startup time now)

but doing it just to control ifunc selection is still
useful (at least for development)

(if eventually there will be widespread use of ifunc
based function multi-versioning then the trap will
not be per process startup but per module load which
is a bit more relevant, but also not something we can
control from the libc)

>> is there something like that on other targets?
> 
> H.J. Lu had a patch to override IFUNCs using (what will now be) a
> tunable but that patch did not make progress.  I hope it will now since
> I too am interested in overriding IFUNC selection using tunables.  But
> then this would be an orthogonal discussion to avoiding the trap since
> the goals of both are different.

i see.