linux-kernel - Re: [PATCH] x86: Fix AMD K6 indirect call check

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Date:	Sun, 21 Apr 2013 11:58:43 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] x86: Fix AMD K6 indirect call check


* Andi Kleen <andi@...stfloor.org> wrote:

> From: Andi Kleen <ak@...ux.intel.com>
> 
> The AMD K6 errata check relies on timing a indirect call.
> But the way it was written it could be optimized to a direct call.
> Force gcc to actually do a indirect call and not just
> constant resolve the target address.
> 
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> 
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index 4a549db..11ea6f6 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -115,7 +115,7 @@ static void __cpuinit init_amd_k6(struct cpuinfo_x86 *c)
>  		 */
>  
>  		n = K6_BUG_LOOP;
> -		f_vide = vide;
> +		asm("" : "=g" (f_vide) : "0" (vide));
>  		rdtscl(d);
>  		while (n--)
>  			f_vide();

Would be useful to read in the changelog about how you found the bug:

 - saw miscalculated values and figured out the reason
 - saw a new warning in GCC
 - saw it during review

?

	Ingo
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