From: John Kacur <jkacur@rogers.com>
To: Linus Torvalds <torvalds@osdl.org>
Cc: Davide Libenzi <davidel@xmailserver.org>,
Jesse Allen <the3dfxdude@gmail.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Andrew Morton <akpm@osdl.org>
Subject: Re: minor nit with decoding popf instruction - was Re: ptrace single-stepping change breaks Wine
Date: Sat, 08 Jan 2005 00:05:48 -0500 [thread overview]
Message-ID: <1105160748.5935.32.camel@linux.site> (raw)
In-Reply-To: <Pine.LNX.4.58.0501062238050.2272@ppc970.osdl.org>
On Fri, 2005-01-07 at 01:48, Linus Torvalds wrote:
> On Thu, 6 Jan 2005, John Kacur wrote:
> >
> > In order to avoid false positives, I think you should remove the line
> > case 0xf0: case 0xf2: case 0xf3:
>
> False positives are ok with instructions that trap - if it traps, we won't
> much care, since the debugger will get notified about that separately (and
> unambiguously).
>
True, I never thought of that, however, you still can drop the test,
there's no point continuing if you detected a lock prefix.
> Also:
>
> > 0xf0 corresponds to the lock prefix which would trigger an invalid
> > opcode exception with a popf instruction.
> >
> > 0xf2 and 0xf3 correspond to the repeat prefixes and are also not valid
> > with popf
>
> I don't think either of those are necessarily true on older x86 chips.
> Nonsensical prefixes used to be silently ignored. Only in later chips has
> Intel been more strict about them, and given them meanings.
>
> In fact, I'm pretty sure it's only "lock" that Intel got a lot more
> careful about. Try it. I'm pretty sure a "rep" prefix is still accepted
> in front of pretty much all instructions. It just doesn't do anything.
>
> Is it used? Probably not. But people have done some strange things to
> "mark" instructions (ie for things like run-time exception handling you
> can "mark" an instruction by prefixing it with a nonsensical "rep" prefix:
> the CPU won't care, and you can check at trap time whether it was one of
> your magic instructions.
>
> Of course, I'd never admit to doing anything that obscure. Never.
>
> Linus
You're right, in practice I've never seen a processor trigger an
exception when it encounters a nonsensical rep prefix, so dropping the
tests for 0xf2 and 0xf3 could potentially miss cases in the unlikely
event that a compiler generated them. Not relevant here, but that
strange technique to mark the instructions would backfire on 128-bit and
64-bit media instructions that use 0xf2 and 0xf3 in the encoding.
John
next prev parent reply other threads:[~2005-01-08 5:16 UTC|newest]
Thread overview: 101+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <Pine.LNX.4.58.0411151439270.2222@ppc970.osdl.org>
2004-11-15 22:53 ` ptrace single-stepping change breaks Wine Roland McGrath
2004-11-19 19:00 ` Eric Pouech
2004-11-19 19:20 ` Linus Torvalds
2004-11-19 19:33 ` Eric Pouech
2004-11-19 19:51 ` Linus Torvalds
2004-11-19 20:41 ` Eric Pouech
2004-11-19 21:22 ` Linus Torvalds
2004-11-19 21:23 ` Daniel Jacobowitz
2004-11-19 21:53 ` Linus Torvalds
2004-11-20 21:49 ` Jesse Allen
2004-11-21 4:55 ` Jesse Allen
2004-11-21 21:32 ` Davide Libenzi
2004-11-21 22:33 ` Linus Torvalds
2004-11-21 23:14 ` Davide Libenzi
2004-11-22 1:12 ` Linus Torvalds
2004-11-22 0:13 ` Andreas Schwab
2004-11-22 1:07 ` Linus Torvalds
2004-11-22 4:06 ` Davide Libenzi
2004-11-22 4:29 ` Linus Torvalds
2004-11-22 6:23 ` Linus Torvalds
2004-11-22 11:06 ` Andreas Schwab
2004-11-22 16:27 ` Linus Torvalds
2004-11-22 13:46 ` Davide Libenzi
2004-11-22 23:15 ` Jesse Allen
2004-11-22 23:48 ` Jesse Allen
2004-11-28 17:01 ` Eric Pouech
2004-11-22 20:52 ` Eric Pouech
2004-11-22 21:10 ` Linus Torvalds
2004-11-22 22:19 ` Mike Hearn
2004-11-22 22:25 ` Linus Torvalds
2004-12-29 2:14 ` Thomas Sailer
2004-12-29 15:02 ` Mike Hearn
2004-12-29 18:53 ` Linus Torvalds
2004-12-29 19:40 ` Jesse Allen
2004-12-29 20:04 ` Linus Torvalds
2004-12-29 21:43 ` Jesse Allen
2004-12-30 0:44 ` Linus Torvalds
2004-12-30 1:13 ` Davide Libenzi
2004-12-30 1:55 ` Linus Torvalds
2004-12-30 4:51 ` Linus Torvalds
2004-12-30 4:58 ` Linus Torvalds
2004-12-30 5:07 ` Davide Libenzi
2004-12-30 7:26 ` Linus Torvalds
2004-12-30 17:59 ` Davide Libenzi
2004-12-30 18:16 ` Linus Torvalds
2004-12-30 19:27 ` Jesse Allen
2004-12-30 19:34 ` Linus Torvalds
2004-12-30 22:46 ` Linus Torvalds
2004-12-30 23:00 ` Daniel Jacobowitz
2004-12-30 23:17 ` Linus Torvalds
2004-12-31 5:36 ` Daniel Jacobowitz
2004-12-31 5:47 ` Linus Torvalds
2004-12-31 7:00 ` Jesse Allen
2004-12-31 15:10 ` Daniel Jacobowitz
2004-12-31 17:19 ` Linus Torvalds
2005-01-01 23:20 ` Daniel Jacobowitz
2005-01-29 9:25 ` Kari Hurtta
2004-12-30 23:15 ` Andi Kleen
2004-12-31 0:38 ` Linus Torvalds
2004-12-31 12:35 ` Andi Kleen
2004-12-31 15:16 ` Davide Libenzi
2004-12-31 17:30 ` Linus Torvalds
2004-12-31 19:55 ` Jesse Allen
2004-12-31 17:14 ` Linus Torvalds
2004-12-31 4:55 ` Jesse Allen
2004-12-31 5:05 ` Linus Torvalds
2004-12-31 5:38 ` Daniel Jacobowitz
2004-12-30 19:19 ` Davide Libenzi
2004-12-30 5:06 ` Davide Libenzi
2004-12-30 4:28 ` Jesse Allen
2004-12-29 20:56 ` Jesse Allen
2004-12-29 19:35 ` Thomas Sailer
2004-12-29 20:13 ` Jesse Allen
2004-12-30 1:49 ` Thomas Sailer
2004-12-30 2:10 ` Linus Torvalds
2004-12-30 2:39 ` Thomas Sailer
2004-12-30 2:57 ` Thomas Sailer
2004-12-30 3:15 ` Thomas Sailer
2004-12-30 4:15 ` Andrew Morton
2004-12-30 10:09 ` Thomas Sailer
2004-12-30 13:06 ` Mike Hearn
2004-12-31 13:13 ` Thomas Sailer
2004-12-31 13:31 ` Mike Hearn
2004-12-31 15:42 ` Jesse Allen
2004-12-31 15:56 ` Davide Libenzi
2004-12-31 15:59 ` Jesse Allen
2004-12-31 22:01 ` Linus Torvalds
2005-01-01 22:04 ` Davide Libenzi
2005-01-01 22:14 ` Linus Torvalds
2005-01-02 3:46 ` Davide Libenzi
2005-01-07 4:51 ` minor nit with decoding popf instruction - was " John Kacur
2005-01-07 6:48 ` Linus Torvalds
2005-01-08 5:05 ` John Kacur [this message]
2004-12-31 15:51 ` Thomas Sailer
[not found] ` <1104873315.3557.87.camel@littlegreen>
2005-01-04 21:21 ` Andrew Morton
2005-01-05 10:43 ` Thomas Sailer
2005-01-05 11:24 ` Ingo Molnar
2005-01-05 11:40 ` Alexandre Julliard
2004-12-30 12:11 ` Mike Hearn
2004-11-20 3:40 ` Roland McGrath
2004-11-19 20:59 ` Grzegorz Kulewski
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