linux-kernel - Re: Simplfying copy_siginfo_to

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Date:   Mon, 24 Jul 2017 10:43:34 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Oleg Nesterov <oleg@...hat.com>,
        Andrei Vagin <avagin@...tuozzo.com>,
        Thomas Gleixner <tglx@...utronix.de>, Greg KH <greg@...ah.com>,
        Andrey Vagin <avagin@...nvz.org>,
        Serge Hallyn <serge@...lyn.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Willy Tarreau <w@....eu>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: Simplfying copy_siginfo_to_user

On Sat, Jul 22, 2017 at 1:25 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> I played with some clever changes such as limiting the copy to 48 bytes,
> disabling the memset and the like but I could not get a strong enough
> signal to say that any one change removed the extra or a clear part of
> it 20ns.

What CPU did you use? Because the SMAP bit in particular matters.

The field-by-field copies are extremely slow on modern CPU's that
implement SMAP, unless you also use the special "unsafe_put_user()"
code (or the nasty old put_user_ex() code that some of the x86 signal
code uses).

So one of the advantages of just copy_to_user() ends up being visible
only on Broadwell+ (or whatever the SMAP cutoff is).

                 Linus