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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Richard Henderson <rth@twiddle.net>,
	Russell King <linux@armlinux.org.uk>,
	Will Deacon <will.deacon@arm.com>,
	Haavard Skinnemoen <hskinnemoen@gmail.com>,
	Steven Miao <realmz6@gmail.com>,
	Jesper Nilsson <jesper.nilsson@axis.com>,
	Mark Salter <msalter@redhat.com>,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	Richard Kuo <rkuo@codeaurora.org>,
	Tony Luck <tony.luck@intel.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	James Hogan <james.hogan@imgtec.com>,
	Michal Simek <monstr@monstr.eu>,
	David Howells <dhowells@redhat.com>,
	Ley Foon Tan <lftan@altera.com>,
	Jonas Bonn <Jonas.Nilsson@synopsys.com>
Subject: Re: [RFC][CFT][PATCHSET v1] uaccess unification
Date: Wed, 29 Mar 2017 17:27:40 -0700	[thread overview]
Message-ID: <CA+55aFyGwYwdk8i7-GbXV7NLTn38e-bow3VD-hHcQmTr9ebAjw@mail.gmail.com> (raw)
In-Reply-To: <09ead054-f62a-76e2-88e0-8d18592d2604@synopsys.com>

On Wed, Mar 29, 2017 at 5:02 PM, Vineet Gupta
<Vineet.Gupta1@synopsys.com> wrote:
>
> I guess I can in next day or two - but mind you the inline version for ARC is kind
> of special vs. other arches. We have this "manual" constant propagation to elide
> the unrolled LD/ST for 1-15 byte stragglers, when @sz is constant.

I don't think that's special. We do that on x86 too, and I suspect ARC
copied it from there (or from somebody else who did it).

But at least on x86 is is limited entirely to the "__" versions, and
it's almost entirely pointless. We actually removed some of that kind
of code because it was *do* pointless, and it had just been copied
around into the "atomic" versions too.

See for example commit bd28b14591b9 ("x86: remove more uaccess_32.h
complexity"), which did that.

The basic "__" versions still do that constant-size thing, but they
really are questionable. Exactly because it's just the "__" versions -
the *regular* "copy_to/from_user()" is an unconditional function call,
because inlining it isn't just the access operations, it's the size
check, and on modern x86 it's also the "set AC to mark the user access
as safe".

.. and many distros enable some of the might_sleep() debugging code
etc. With any of that, inlining is simply a *bad* choice.

                  Linus

  reply	other threads:[~2017-03-30  0:27 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-03-29  5:57 [RFC][CFT][PATCHSET v1] uaccess unification Al Viro
2017-03-29 20:08 ` Vineet Gupta
2017-03-29 20:29   ` Al Viro
2017-03-29 20:37     ` Linus Torvalds
2017-03-29 21:03       ` Al Viro
2017-03-29 21:24         ` Linus Torvalds
2017-03-29 23:09           ` Al Viro
2017-03-29 23:43             ` Linus Torvalds
2017-03-30 15:31               ` Al Viro
2017-03-29 21:14     ` Vineet Gupta
2017-03-29 23:42       ` Al Viro
2017-03-30  0:02         ` Vineet Gupta
2017-03-30  0:27           ` Linus Torvalds [this message]
2017-03-30  1:15             ` Al Viro
2017-03-30 20:40             ` Vineet Gupta
2017-03-30 20:59               ` Linus Torvalds
2017-03-30 23:21                 ` Russell King - ARM Linux
2017-03-30 12:32 ` Martin Schwidefsky
2017-03-30 14:48   ` Al Viro
2017-03-30 16:22 ` Russell King - ARM Linux
2017-03-30 16:43   ` Al Viro
2017-03-30 17:18     ` Linus Torvalds
2017-03-30 18:48       ` Al Viro
2017-03-30 18:54         ` Al Viro
2017-03-30 18:59           ` Linus Torvalds
2017-03-30 19:10             ` Al Viro
2017-03-30 19:19               ` Linus Torvalds
2017-03-30 21:08                 ` Al Viro
2017-03-30 18:56         ` Linus Torvalds
2017-03-31  0:21 ` Kees Cook
2017-03-31 13:38   ` James Hogan
2017-04-03 16:27 ` James Morse
2017-04-04 20:26 ` Max Filippov
2017-04-04 20:52   ` Al Viro
2017-04-05  5:05 ` ia64 exceptions (Re: [RFC][CFT][PATCHSET v1] uaccess unification) Al Viro
2017-04-05  8:08   ` Al Viro
2017-04-05 18:44     ` Tony Luck
2017-04-05 20:33       ` Al Viro
2017-04-07  0:24 ` [RFC][CFT][PATCHSET v2] uaccess unification Al Viro
2017-04-07  0:35   ` Al Viro
     [not found] <CACVxJT8+fQqvpSPb9rTWFy6g7moqUqxi+Ewjcg0ykuqo=vm4Ow@mail.gmail.com>
2017-03-30 13:27 ` [RFC][CFT][PATCHSET v1] " Alexey Dobriyan

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