linux-kernel - Re: Re: [PATCH v2 1/2] x86/cpu: Introduce x86_get_cpufreq

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Message-ID: <947de021-df91-9219-7378-8addc6f66612@bytedance.com>
Date:   Fri, 3 Dec 2021 15:34:04 +0800
From:   zhenwei pi <pizhenwei@...edance.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: Re: [PATCH v2 1/2] x86/cpu: Introduce x86_get_cpufreq_khz()

On 12/3/21 6:25 AM, Peter Zijlstra wrote:
> On Wed, Dec 01, 2021 at 10:46:49AM +0800, zhenwei pi wrote:
>> Wrapper function x86_get_cpufreq_khz() to get frequency on a x86
>> platform, hide detailed implementation from proc routine.
>>
>> Also export this function for the further use, a typical case is that
>> kvm module gets the frequency of the host and tell the guest side by
>> kvmclock.
> 
> This function was already complete crap, and now you want to export it,
> *WHY* ?!
> 
> What possible use does KVM have for this random number generator?
> 
A KVM guest overwrites the '.calibrate_tsc' and '.calibrate_cpu' if 
kvmclock is supported:

in function kvmclock_init(void) (linux/arch/x86/kernel/kvmclock.c)
	...
         x86_platform.calibrate_tsc = kvm_get_tsc_khz;
         x86_platform.calibrate_cpu = kvm_get_tsc_khz;
	...

And kvm_get_tsc_khz reads PV data from host side. Before guest reads 
this, KVM should writes the frequency into the PV data structure.

And the problem is that KVM gets tsc_khz directly without aperf/mperf 
detection. So user may gets different frequency(cat /proc/cpuinfo) from 
guest & host.

Or is that possible to export function 'aperfmperf_get_khz'?

-- 
zhenwei pi