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

H. Peter Anvin
2010-08-17T06:30:55Z

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@vmware.com>
> Cc: H. Peter Anvin <hpa@zytor.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.