linux-kernel - Re: [PATCHv6 23/36] x86/vdso: Allocate timens vdso

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Date:   Fri, 16 Aug 2019 08:23:07 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@...il.com>,
        Adrian Reber <adrian@...as.de>,
        Andrei Vagin <avagin@...nvz.org>,
        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>,
        Jann Horn <jannh@...gle.com>, Jeff Dike <jdike@...toit.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        linux-api@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCHv6 23/36] x86/vdso: Allocate timens vdso

On 8/15/19 9:38 AM, 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.
> 
> The propose is to allocate a second vdso code with dynamically
> patched out (disabled by static_branch) timens code on boot time.
> 
> Allocate another vdso and copy original code.


I'm unconvinced that any of this magic is wise.  I think you should make 
a special timens vvar page that causes the normal fastpath to fail 
(using a special vclock mode, a special seq value, or a special "last" 
value) and then make the failure path detect that timens is in use and 
use the timens path.


--Andy