linux-kernel - Re: [PATCHv6 25/36] vdso: Introduce vdso_static_branch

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Date:   Thu, 15 Aug 2019 20:03:02 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dmitry Safonov <dima@...sta.com>
cc:     linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Andrei Vagin <avagin@...il.com>,
        Adrian Reber <adrian@...as.de>,
        Andrei Vagin <avagin@...nvz.org>,
        Andy Lutomirski <luto@...nel.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>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        linux-api@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCHv6 25/36] vdso: Introduce vdso_static_branch_unlikely()

On Thu, 15 Aug 2019, Dmitry Safonov wrote:

> From: Andrei Vagin <avagin@...il.com>
> 
> As it has been discussed on timens RFC, adding a new conditional branch
> `if (inside_time_ns)` on VDSO for all processes is undesirable.
> 
> 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.
> 
> Introduce vdso_static_branch_unlikely(), which is similar to
> static_branch_unlikely(); alias it with timens_static_branch_unlikely()
> under CONFIG_TIME_NS.
> 
> The timens code in vdso will look like this:
> 
>    if (timens_static_branch_unlikely()) {
> 	   clk_to_ns(clk, ts);

Please name that clk_to_namespace(). _ns() is widely used for nanoseconds.

>    }
> 
> The version of vdso which is compiled from sources will never execute
> clk_to_ns(). And then we can patch the 'no-op' in the straight-line
> codepath with a 'jump' instruction to the out-of-line true branch and
> get the timens version of the vdso library.

Colour me confused. Why do we need that static branch at all?

Why don't we compile VDSO_NO_NAMESPACE and VDSO_NAMESPACE right away? One
has the clk_to_namespace() one does not. The you can spare the whole static
key patching and the NO_NAMESPACE variant does not have extra 5 NOPS.

The VDSO is one page IIRC, so having the extra namespace variant around
does really not matter at all.

Thanks,

	tglx