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

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Message-ID: <3f0ac089-387e-ab58-59e9-5f8cafa1573d@gmail.com>
Date:   Fri, 8 Feb 2019 15:18:56 +0000
From:   Dmitry Safonov <0x7f454c46@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Dmitry Safonov <dima@...sta.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Adrian Reber <adrian@...as.de>,
        Andrei Vagin <avagin@...nvz.org>,
        Andrei Vagin <avagin@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andy Tucker <agtucker@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jeff Dike <jdike@...toit.com>, Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        linux-api@...r.kernel.org, x86@...nel.org,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH 16/32] x86/vdso: Generate vdso{,32}-timens.lds

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