Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20190729215758.28405-1-dima@arista.com> <20190729215758.28405-29-dima@arista.com>
In-Reply-To: <20190729215758.28405-29-dima@arista.com>
From:   Andy Lutomirski <luto@kernel.org>
Date:   Wed, 31 Jul 2019 22:21:33 -0700
Message-ID: <CALCETrXL6pzDoHfn9Niw_CxNX-_W3=yotDYuqK+kxPhOLofmNA@mail.gmail.com>
Subject: Re: [PATCHv5 28/37] x86/vdso: Enable static branches for the timens vdso
To:     Dmitry Safonov <dima@arista.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        Andrei Vagin <avagin@gmail.com>,
        Adrian Reber <adrian@lisas.de>,
        Andrei Vagin <avagin@openvz.org>,
        Andy Lutomirski <luto@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Cyrill Gorcunov <gorcunov@openvz.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Jann Horn <jannh@google.com>, Jeff Dike <jdike@addtoit.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Pavel Emelyanov <xemul@virtuozzo.com>,
        Shuah Khan <shuah@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Linux Containers <containers@lists.linux-foundation.org>,
        criu@openvz.org, Linux API <linux-api@vger.kernel.org>,
        X86 ML <x86@kernel.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Jul 29, 2019 at 2:58 PM Dmitry Safonov <dima@arista.com> wrote:
>
> From: Andrei Vagin <avagin@gmail.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.
>
> The timens code in vdso looks like this:
>
> if (timens_static_branch_unlikely()) {
>        clk_to_ns(clk, ts);
> }

I'm confused.  Now we effectively have *three* versions: the vDSO
without timens, and vDSO with timens but with it switched off, and the
vDSO with timens on.  This seems like too much.

What you need is, IMO, a static-branch-ish thing that is per mm.  This
has a fundamental problem that the vDSO can be modified using
FOLL_FORCE.  Perhaps any CoW of the vDSO should implicitly switch the
static branch on, which at least gives some degree of correctness even
if it's a bit surprising.

--Andy