linux-kernel - Re: [PATCH 4/4] optimize and simplify get_cycles

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Message-Id: <20070306162541.2b8a148d.akpm@linux-foundation.org>
Date:	Tue, 6 Mar 2007 16:25:41 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Joerg Roedel" <joerg.roedel@....com>
Cc:	discuss@...-64.org, "Andi Kleen" <ak@...e.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] optimize and simplify get_cycles_sync()

On Wed, 28 Feb 2007 15:25:54 +0100
"Joerg Roedel" <joerg.roedel@....com> wrote:

> From: Joerg Roedel <joerg.roedel@....com>
> 
> This patch simplifies the get_cycles_sync() function by removing the
> #ifdefs from it. Further it introduces an optimization for AMD
> processors. There the RDTSCP instruction is used instead of CPUID;RDTSC
> which is helpfull if the kernel runs as a KVM guest. Running as a guest
> makes CPUID very expensive because it causes an intercept of the guest.

Problem:

http://test.kernel.org/functional/index.html

lots of builds fell over because their binutils versions don't understand
rdtscp.

I don't know how recently rdtscp support was added to binutils, but
this is likely to be a problem.  Perhaps we can change this patch to
embed the hex code for that opcode instead?
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