Re: Re: [PATCH v2 2/2] KVM: x86: use x86_get_freq to get freq for kvmclock

zhenwei pi
2021-12-02T05:30:18Z

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