linux-kernel - Re: questions about rd{msr|tsc|pmc} instruction with x86-64

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Message-ID: <48F6E1A7.2070602@zytor.com>
Date:	Wed, 15 Oct 2008 23:39:35 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jike Song <albcamus@...il.com>
CC:	gcosta@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: questions about rd{msr|tsc|pmc} instruction with x86-64

Jike Song wrote:
> 
> By my reading of Intel & AMD manuals, this comment is wrong.  rdmsr of
> x86-64 has the same behavior as i386, namely the high 32bit returns in
> edx and the low 32bit in eax, not "returns at rax".  And the gcc
> constraint "A" does mean edx:eax in x86-64 also, at least when testing
> on my AMD Turion 64 processor.
> 

"A" means rdx:rax on x86-64, not edx:eax.  For a 64-bit number, it 
means, literally, "one of rdx or rax"!  As you correctly point out, this 
is not how rdmsr works.

	-hpa
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