linux-kernel - Re: [RFT] Port 0x80 I/O speed

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Date:	Wed, 12 Dec 2007 11:25:52 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Kyle McMartin <kyle@...artin.ca>
CC:	Rene Herman <rene.herman@...il.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>, dpreed@...d.com,
	Alan Cox <alan@...rguk.ukuu.org.uk>, pavel@....cz,
	andi@...stfloor.org, rol@...917.net,
	Krzysztof Halasa <khc@...waw.pl>, david@...idnewall.com,
	john@...ffel.org, linux-os@...logic.com
Subject: Re: [RFT] Port 0x80 I/O speed

Kyle McMartin wrote:
> On Wed, Dec 12, 2007 at 12:31:18AM +0100, Rene Herman wrote:
>> 	asm volatile ("rdtsc": "=A" (tsc));
> 
> rdtsc returns a 64-bit value in two 32-bit regs, you need to do
> 
> inline unsigned long long rdtsc(void)
> {
> 	unsigned int lo, hi;
> 	asm volatile ("rdtsc": "=a" (lo), "=d" (hi));
> 	return (unsigned long long)hi << 32 | lo;
> }
> 
> as in msr.h, otherwise you'll only be looking at the value in %rax.
> 

"=A" works on 32-bit systems (only), obviously, and gcc will generally 
produce slightly better code as a result (gcc could really use a 
register renaming/copy propagation step *after* multi-register entities 
are broken apart, at least on architectures which don't have register 
pairs as a hardware constraint.)

	-hpa
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