linux-kernel - Re: [Patch] Skip cpu_calibrate for kernel running under hypervisors.

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Date:	Mon, 16 Aug 2010 23:30:48 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	akataria@...are.com
CC:	the arch/x86 maintainers <x86@...nel.org>,
	Greg KH <gregkh@...e.de>, "greg@...ah.com" <greg@...ah.com>,
	"ksrinivasan@...ell.com" <ksrinivasan@...ell.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Patch] Skip cpu_calibrate for kernel running under	hypervisors.

On 08/16/2010 10:51 PM, Alok Kataria wrote:
>>
>> I'm somewhat reluctant to take this one, since it assumes all the
>> hypervisors act the same.  This seems rather inherently wrong.  In fact,
>> the whole statement is fishy as heck... instead of being dependent on
>> AMD and so on, 
> 
> The check about being on AMD is something that was already there. 
> 

I know it was... and calibrate_cpu() seems to be an AMD-specific
function, but that's rather crappy.  I'm thinking that perhaps we should
make it an x86_init function, then the AMD CPU detection can install it
and the vmware hypervisor detection can uninstall it.

>> this should either be a function pointer or a CPU
>> (mis)feature bit.
> 
> In any case, I agree that my previous patch did assume all hypervisors
> to be same, which might be wrong. How about using the
> X86_FEATURE_TSC_RELIABLE bit for this too ? i.e. Skip cpu_calibrate call
> if TSC_RELIABLE bit is set. As of now that bit is set for vmware only. 
> 
> Something like the below.
> 
> Signed-off-by: Alok N Kataria <akataria@...are.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> 
> Index: linux-x86-tree.git/arch/x86/kernel/tsc.c
> ===================================================================
> --- linux-x86-tree.git.orig/arch/x86/kernel/tsc.c	2010-08-03 12:21:20.000000000 -0700
> +++ linux-x86-tree.git/arch/x86/kernel/tsc.c	2010-08-16 21:59:32.000000000 -0700
> @@ -927,7 +927,8 @@ void __init tsc_init(void)
>  	}
>  
>  	if (cpu_has(&boot_cpu_data, X86_FEATURE_CONSTANT_TSC) &&
> -			(boot_cpu_data.x86_vendor == X86_VENDOR_AMD))
> +	    (boot_cpu_data.x86_vendor == X86_VENDOR_AMD) &&
> +	    !(cpu_has(&boot_cpu_data, X86_FEATURE_TSC_RELIABLE)))
>  		cpu_khz = calibrate_cpu();
>  
>  	printk("Detected %lu.%03lu MHz processor.\n",
> 

That seems like a much better approach.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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