From: Jamie Lokier <jamie@shareable.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Scott Lurndal <scott.lurndal@3leafsystems.com>,
David Howells <dhowells@redhat.com>,
mingo@elte.hu, tglx@linutronix.de, linux-arch@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] X86: Optimise fls(), ffs() and fls64()
Date: Tue, 6 Apr 2010 14:57:32 +0100 [thread overview]
Message-ID: <20100406135732.GC24003@shareable.org> (raw)
In-Reply-To: <alpine.LFD.2.00.1003261038160.3721@i5.linux-foundation.org>
Linus Torvalds wrote:
> On Fri, 26 Mar 2010, Scott Lurndal wrote:
> >
> > I wonder if Intel's EM64 stuff makes this more deterministic, perhaps
> > David's implementation would work for x86_64 only?
>
> Limiting it to x86-64 would certainly remove all the worries about all the
> historical x86 clones.
>
> I'd still worry about it for future Intel chips, though. I absolutely
> _detest_ relying on undocumented features - it pretty much always ends up
> biting you eventually. And conditional writeback is actually pretty nasty
> from a microarchitectural standpoint.
On the same subject of relying on undocumented features:
/* If SMP and !X86_PPRO_FENCE. */
#define smp_rmb() barrier()
I've seen documentation, links posted to lkml ages ago, which implies
this is fine on 64-bit for both Intel and AMD.
But it appears to be relying on undocumented behaviour on 32-bit...
Are you sure it is ok? Has anyone from Intel/AMD ever confirmed it is
ok? Has it been tested? Clones?
-- Jamie
next prev parent reply other threads:[~2010-04-06 13:57 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-03-26 14:42 [PATCH 1/3] X86: Optimise fls(), ffs() and fls64() David Howells
2010-03-26 14:42 ` [PATCH 2/3] Adjust the comment on get_order() to describe the size==0 case David Howells
2010-03-26 14:42 ` [PATCH 3/3] Optimise get_order() David Howells
2010-03-26 17:23 ` [PATCH 1/3] X86: Optimise fls(), ffs() and fls64() Linus Torvalds
2010-03-26 17:37 ` Scott Lurndal
2010-03-26 17:42 ` Linus Torvalds
2010-04-06 13:57 ` Jamie Lokier [this message]
2010-04-06 14:40 ` Linus Torvalds
2010-03-26 17:42 ` David Howells
2010-03-26 17:45 ` Linus Torvalds
2010-03-26 17:58 ` Ralf Baechle
2010-03-26 18:03 ` Linus Torvalds
2010-03-26 18:16 ` Matthew Wilcox
2010-04-06 13:30 ` Matthew Wilcox
2010-04-14 11:49 ` David Howells
2010-04-14 14:30 ` Avi Kivity
2010-04-15 8:48 ` David Howells
2010-04-15 8:49 ` Avi Kivity
2010-04-15 11:41 ` Jamie Lokier
2010-03-26 17:52 ` Matthew Wilcox
[not found] ` <4BACCB4E.7010108@draigBrady.com>
2010-04-14 13:13 ` David Howells
-- strict thread matches above, loose matches on Subject: below --
2010-01-13 19:39 David Howells
2010-01-13 20:15 ` Geert Uytterhoeven
2010-01-13 21:59 ` David Howells
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