linux-kernel - Re: [PATCH] x86_64: check if Tom2 is enabled

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Date:	Mon, 21 Jan 2008 19:46:15 +0100
From:	Andi Kleen <ak@...e.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	Yinghai Lu <Yinghai.Lu@....com>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Jesse Barnes <jesse.barnes@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] x86_64: check if Tom2 is enabled


> 
> Actually, I think it depends on the specific MSR - some use the halves 
> for different data, whereas others treat it as a large 64-bit object.

Even if there are different fields in there it is still much cleaner
and simpler if there is only a single number to manipulate.

> Ironically enough, the way the MSR interfaces were carried into the 
> 64-bit world makes the situation worse on 64 bits; edx:eax is the common 
> way to represent a 64-bit value on 32 bits.

It's merely an implementation detail of the instruction. But even on 32bit
there is about zero reason to expose that to C code. rdmsr/wrmsr et.al. should
have been defined as 64bit only interface in Linux from day zero.

-Andi
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