linux-kernel - Re: [PATCH 04/18] Make cpu_tsc

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Message-ID: <4C461BAF.90804@redhat.com>
Date:	Tue, 20 Jul 2010 11:57:03 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Avi Kivity <avi@...hat.com>
CC:	KVM <kvm@...r.kernel.org>, Marcelo Tosatti <mtosatti@...hat.com>,
	Glauber Costa <glommer@...hat.com>,
	Linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/18] Make cpu_tsc_khz updates use local CPU

On 07/19/2010 10:53 PM, Avi Kivity wrote:
> On 07/19/2010 11:06 PM, Zachary Amsden wrote:
>>>> +static void tsc_khz_changed(void *data)
>>>>   {
>>>> -    /* nothing */
>>>> +    struct cpufreq_freqs *freq = data;
>>>> +    unsigned long khz = 0;
>>>> +
>>>> +    if (data)
>>>> +        khz = freq->new;
>>>> +    else if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
>>>> +        khz = cpufreq_quick_get(raw_smp_processor_id());
>>>> +    if (!khz)
>>>> +        khz = tsc_khz;
>>>> +    __get_cpu_var(cpu_tsc_khz) = khz;
>>>>   }
>>>
>>> Do we really need to cache cpufreq_quick_get()?  If it's really 
>>> quick, why not just use it everywhere instead of cacheing it?  Not a 
>>> comment on this patch.
>>>
>>
>>
>> If cpufreq is compiled in, but disabled, it returns zero, so we need 
>> some sort of logic.
>
> Maybe it's better to put it into cpufreq_quick_get().  Inconsistent 
> APIs that appear to work are bad.
>

I don't think it's quite so simple; cpufreq is platform independent and 
tsc_khz is a platform specific export.  It seems cpufreq is designed to 
return zero when disabled and we're the unusual ones for wanting to use it.

Zach
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