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

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Date:	Wed, 12 Dec 2007 18:41:15 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Rene Herman <rene.herman@...il.com>
CC:	Kyle McMartin <kyle@...artin.ca>,
	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

Rene Herman wrote:
> On 12-12-07 06:23, 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.
> 
> On 32-bit, "=A" is edx:eax. Not sure what the point is in not letting it 
> be that on 64-bit in fact, but yes, the thing should be compiled as 32-bit.

On 64-bit, "=A" is rdx:rax, which means that a 64-bit value ends up in 
rax only (a 128-bit value, which gcc does support on 128-bit platforms, 
would be in rdx:rax.)

gcc can't deal internally with values that span partial registers.

	-hpa
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