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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 24 Jul 2017 10:43:34 -0700
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Subject: Re: Simplfying copy_siginfo_to_user
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>, Al Viro <viro@zeniv.linux.org.uk>,
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On Sat, Jul 22, 2017 at 1:25 PM, Eric W. Biederman
<ebiederm@xmission.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