Re: Simplfying copy_siginfo_to_user

From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@kernel.org>,
	Oleg Nesterov <oleg@redhat.com>,
	Andrei Vagin <avagin@virtuozzo.com>,
	Thomas Gleixner <tglx@linutronix.de>, Greg KH <greg@kroah.com>,
	Andrey Vagin <avagin@openvz.org>, Serge Hallyn <serge@hallyn.com>,
	Pavel Emelyanov <xemul@virtuozzo.com>,
	Cyrill Gorcunov <gorcunov@openvz.org>,
	Peter Zijlstra <peterz@infradead.org>, Willy Tarreau <w@1wt.eu>,
	"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Linux Containers <containers@lists.linux-foundation.org>,
	Michael Kerrisk <mtk.manpages@gmail.com>
Subject: Re: Simplfying copy_siginfo_to_user
Date: Tue, 25 Jul 2017 02:37:57 +0100	[thread overview]
Message-ID: <20170725013756.GH2063@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CA+55aFyH5W2doo9vxXta_-pXfNXqQ19d7z48k1hmfAot+aJvMw@mail.gmail.com>

On Mon, Jul 24, 2017 at 10:43:34AM -0700, Linus Torvalds wrote:
> 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).

Guys, could you take a look at vfs.git#work.siginfo?  I'd been pretty
much buried lately (and probably will for several more weeks - long-distance
moves *suck*), so that thing got stalled, but it might be worth a look.

The code generated in copy_siginfo_to_user() in it looks reasonably good,
we don't copy more than we need and all copying to userland is done
by copy_to_user() - one call per call of copy_siginfo_to_user(), so
SMAP crap is not an issue.

The next thing I hope to do is converting compat side of that thing to
the same; that got stalled.

Al "Buried in boxes" Viro...