linux-kernel - Re: [PATCH 1/3] X86: Optimise fls(), ffs() and fls64()

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Message-ID: <20100406135732.GC24003@shareable.org>
Date:	Tue, 6 Apr 2010 14:57:32 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Scott Lurndal <scott.lurndal@...afsystems.com>,
	David Howells <dhowells@...hat.com>, mingo@...e.hu,
	tglx@...utronix.de, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] X86: Optimise fls(), ffs() and fls64()

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
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