Re: [PATCH 16/32] x86/vdso: Generate vdso{,32}-timens.lds

[Dmitry Safonov <0x7f454c46@gmail.com>]

On 2/8/19 9:57 AM, Thomas Gleixner wrote:
> On Thu, 7 Feb 2019, Rasmus Villemoes wrote:
> 
> Cc: + Vincenzo, Will
> 
>> On 06/02/2019 01.10, Dmitry Safonov wrote:
>>> As it has been discussed on timens RFC, adding a new conditional branch
>>> `if (inside_time_ns)` on VDSO for all processes is undesirable.
>>> It will add a penalty for everybody as branch predictor may mispredict
>>> the jump. Also there are instruction cache lines wasted on cmp/jmp.
>>>
>>> Those effects of introducing time namespace are very much unwanted
>>> having in mind how much work have been spent on micro-optimisation
>>> vdso code.
>>>
>>> Addressing those problems, there are two versions of VDSO's .so:
>>> for host tasks (without any penalty) and for processes inside of time
>>> namespace with clk_to_ns() that subtracts offsets from host's time.
>>>
>>> Unfortunately, to allow changing VDSO VMA on a running process,
>>> the entry points to VDSO should have the same offsets (addresses).
>>> That's needed as i.e. application that calls setns() may have already
>>> resolved VDSO symbols in GOT/PLT.
>>
>> These (14-19, if I'm reading them right) seems to add quite a lot of
>> complexity and fragility to the build, and other architectures would
>> probably have to add something similar to their vdso builds.
> 
> Yes and we really want to avoid that. The VDSO implementations are
> pointlessly different accross the architectures and there is effort on the
> way to consolidate them:
> 
>   https://lkml.kernel.org/r/20190115135539.24762-1-vincenzo.frascino@arm.com
> 
> I talked to Vincenzo earlier this week and he's working on a new version of
> that. The timens stuff wants to go on top of the consolidation otherwise we
> end up with another set of pointlessly different and differently broken
> VDSO variants.

That looks awesome!
I've missed the tread about it, will catch the details.

Thanks much,
          Dmitry