Subject: Re: Re: [PATCH v2 2/2] KVM: x86: use x86_get_freq to get freq for
 kvmclock
To:     Thomas Gleixner <tglx@linutronix.de>, pbonzini@redhat.com
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org, x86@kernel.org
References: <20211201024650.88254-1-pizhenwei@bytedance.com>
 <20211201024650.88254-3-pizhenwei@bytedance.com> <877dcn7md2.ffs@tglx>
From:   zhenwei pi <pizhenwei@bytedance.com>
Message-ID: <b37ffc3d-4038-fc5e-d681-b89c04a37b04@bytedance.com>
Date:   Thu, 2 Dec 2021 13:26:27 +0800
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On 12/2/21 10:48 AM, Thomas Gleixner wrote:
> On Wed, Dec 01 2021 at 10:46, zhenwei pi wrote:
>> If the host side supports APERF&MPERF feature, the guest side may get
>> mismatched frequency.
>>
>> KVM uses x86_get_cpufreq_khz() to get the same frequency for guest side.
>>
>> Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
>> ---
>>   arch/x86/kvm/x86.c | 4 +---
>>   1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 5a403d92833f..125ed3c8b21a 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -8305,10 +8305,8 @@ static void tsc_khz_changed(void *data)
>>   
>>   	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;
>> +		khz = x86_get_cpufreq_khz(raw_smp_processor_id());
> 
> my brain compiler tells me that this is broken.
> Without this patch:
1, boot_cpu_has(X86_FEATURE_CONSTANT_TSC) is true:
no kvmclock_cpufreq_notifier, and khz = tsc_khz;

2, boot_cpu_has(X86_FEATURE_CONSTANT_TSC) is false:
during installing kmod, try cpufreq_quick_get(), or use tsc_khz;
and get changed by kvmclock_cpufreq_notifier.

With this patch:
1, boot_cpu_has(X86_FEATURE_CONSTANT_TSC) is true:
no kvmclock_cpufreq_notifier, try aperf/mperf, or try 
cpufreq_quick_get(), or use cpu_khz

2, boot_cpu_has(X86_FEATURE_CONSTANT_TSC) is false:
during installing kmod, try aperf/mperf, or try cpufreq_quick_get(), or 
use cpu_khz;
and get changed by kvmclock_cpufreq_notifier.

I tested on Skylake&Icelake CPU, and got different CPU frequency from 
host & guest, the main purpose of this patch is to get the same frequency.

-- 
zhenwei pi