linux-kernel - Re: x86-64: memset()/memcpy() not fully standards compliant

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Message-ID: <20120106094922.GU11715@one.firstfloor.org>
Date:	Fri, 6 Jan 2012 10:49:22 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Jan Beulich <JBeulich@...e.com>,
	mingo@...e.hu, tglx@...utronix.de, linux-kernel@...r.kernel.org
Subject: Re: x86-64: memset()/memcpy() not fully standards compliant

On Thu, Jan 05, 2012 at 06:03:23PM -0800, H. Peter Anvin wrote:
> On 01/05/2012 05:47 PM, Andi Kleen wrote:
> >>
> >> Is that still true, and do we even use string instructions still on
> >> those old CPUs?  Jan's fixes don't introduce any additional delays in
> >> the non-string-instruction paths.
> > 
> > Yes various of the CPUs with bugs used string instructions.
> > 
> 
> Which CPUs are you talking about here?

This was various iterations of K8 and PSC. Early Meroms may also have
had issues (not 100% sure).

> 
> > 
> > Both string and non string instructions are used on modern CPUs,
> > so making any of that slower is not a good idea.
> > 
> 
> Obviously not, but I'm perfectly fine turning REP_GOOD off on old broken
> CPUs.

That would be even worse.

You would slow a critical fast path operation down for something
that never happens?!?  There were big differences between strings
and the unrolled loop for several.

IMHO it's a war games situation: "the only way to win is not to play"

-Andi

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