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

[zhenwei pi]

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@xxxxxxxxxxxxx>
> > ---
> >   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