linux-kernel - Re: [PATCH 1/1] Speedfreq-SMI call clobbers ECX

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Date:	Wed, 05 Mar 2008 08:02:16 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Stephan Diestelhorst <langer_mann@....de>, davej@...emonkey.org.uk,
	cpufreq@...ts.linux.org.uk, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] Speedfreq-SMI call clobbers ECX

Ingo Molnar wrote:
> * Stephan Diestelhorst <langer_mann@....de> wrote:
> 
>> @@ -184,7 +184,7 @@ static void speedstep_set_state (unsigne
>>  		__asm__ __volatile__(
>>  			"movl $0, %%edi\n"
>>  			"out %%al, (%%dx)\n"
>> -			: "=b" (new_state), "=D" (result)
>> +			: "=b" (new_state), "=D" (result), "=c" (ecx_clobber)
>>  			: "a" (command), "b" (function), "c" (state), "d" (smi_port), "S" (0)
>>  			);
> 
> stupid suggestion: why not do a pusha/popa around those instructions, to 
> make sure everything is restored? This isnt a fastpath and being 
> conservative about SMI side-effects cannot hurt ...
> 

You can't pusha/popa if you expect a result.  You can, of course, push 
and pop individual registers.

It's also kind of odd to do "movl $0,%%edi" instead of just setting EDI 
as an input.

	-hpa
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