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    SubjectRe: [PATCHv5 28/37] x86/vdso: Enable static branches for the timens vdso
    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

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