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From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>,
	Christophe Leroy <christophe.leroy@c-s.fr>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: objtool clac/stac handling change..
Date: Fri, 3 Jul 2020 23:04:07 +0100	[thread overview]
Message-ID: <20200703220407.GU2786714@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20200703215922.GT2786714@ZenIV.linux.org.uk>

On Fri, Jul 03, 2020 at 10:59:22PM +0100, Al Viro wrote:
> On Fri, Jul 03, 2020 at 02:10:08PM -0700, Linus Torvalds wrote:
> > On Fri, Jul 3, 2020 at 2:02 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> > >
> > > Actually, for more serious problem consider arch/x86/lib/copy_user_64.S
> > 
> > What? No.
> > 
> > > In case of an unhandled fault on attempt to read an (unaligned) word,
> > > the damn thing falls back to this:
> > > SYM_CODE_START_LOCAL(.Lcopy_user_handle_tail)
> > >         movl %edx,%ecx
> > > 1:      rep movsb
> > > 2:      mov %ecx,%eax
> > >         ASM_CLAC
> > >         ret
> > >
> > >         _ASM_EXTABLE_UA(1b, 2b)
> > > SYM_CODE_END(.Lcopy_user_handle_tail)
> > 
> > In the case of "we did an unaligned word at the end of a page, we took
> > a fault, and now we have to start all over", the _least_ of our
> > problems is that part of "starting over" would now imply doing a
> > "stac" again.
> 
> What do you mean, start over?  It's picking a few remaining bytes out
> of that word, *not* redoing the entire thing.

I'm _not_ saying that it's a performance-critical place, in case that's not
obvious - just trying to head off potential confusion re what that code is
doing.

  reply	other threads:[~2020-07-03 22:04 UTC|newest]

Thread overview: 47+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-01 18:22 objtool clac/stac handling change Linus Torvalds
2020-07-01 18:29 ` Andy Lutomirski
2020-07-01 19:35   ` Linus Torvalds
2020-07-01 20:36     ` Andy Lutomirski
2020-07-01 20:51       ` Josh Poimboeuf
2020-07-01 21:02         ` Linus Torvalds
2020-07-02  0:00           ` Josh Poimboeuf
2020-07-02  8:05             ` Peter Zijlstra
2020-07-01 20:51       ` Linus Torvalds
2020-07-02  0:47         ` Andy Lutomirski
2020-07-02  2:30           ` Linus Torvalds
2020-07-02  2:35             ` Linus Torvalds
2020-07-02  3:08             ` Andy Lutomirski
2020-07-01 18:41 ` Al Viro
2020-07-01 19:04   ` Linus Torvalds
2020-07-01 19:59     ` Al Viro
2020-07-01 20:25       ` Linus Torvalds
2020-07-02 13:34         ` Michael Ellerman
2020-07-02 14:01           ` Al Viro
2020-07-02 14:04             ` Al Viro
2020-07-02 15:13           ` Christophe Leroy
2020-07-02 20:13             ` Linus Torvalds
2020-07-03  3:59               ` Michael Ellerman
2020-07-03  3:17             ` Michael Ellerman
2020-07-03  5:27               ` Christophe Leroy
2020-07-02 19:52           ` Linus Torvalds
2020-07-02 20:17             ` Al Viro
2020-07-02 20:32               ` Linus Torvalds
2020-07-02 20:59                 ` Al Viro
2020-07-02 21:55                   ` Linus Torvalds
2020-07-03  1:33                     ` Al Viro
2020-07-03  3:32                       ` Linus Torvalds
2020-07-03 21:02                       ` Al Viro
2020-07-03 21:10                         ` Linus Torvalds
2020-07-03 21:41                           ` Andy Lutomirski
2020-07-03 22:25                             ` Al Viro
2020-07-03 21:59                           ` Al Viro
2020-07-03 22:04                             ` Al Viro [this message]
2020-07-03 22:12                           ` Al Viro
2020-07-04  0:49                         ` Al Viro
2020-07-04  1:54                           ` Linus Torvalds
2020-07-04  2:30                             ` Al Viro
2020-07-04  3:06                               ` Linus Torvalds
2020-07-04  2:11                           ` Al Viro
2020-07-07 12:35                             ` David Laight
2020-07-10 22:37                               ` Linus Torvalds
2020-07-13  9:32                                 ` David Laight

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