答复: [PATCH][v5] KVM: X86: support APERF/MPERF registers

["Li,Rongqing" <lirongqing@baidu.com>]

> -----邮件原件-----
> 发件人: Xiaoyao Li [mailto:xiaoyao.li@intel.com]
> 发送时间: 2020年5月30日 18:40
> 收件人: Li,Rongqing <lirongqing@baidu.com>; linux-kernel@vger.kernel.org;
> kvm@vger.kernel.org; x86@kernel.org; hpa@zytor.com; bp@alien8.de;
> mingo@redhat.com; tglx@linutronix.de; jmattson@google.com;
> wanpengli@tencent.com; vkuznets@redhat.com;
> sean.j.christopherson@intel.com; pbonzini@redhat.com;
> wei.huang2@amd.com
> 主题: Re: [PATCH][v5] KVM: X86: support APERF/MPERF registers
> 
> On 5/30/2020 12:35 PM, Li RongQing wrote:
> > Guest kernel reports a fixed cpu frequency in /proc/cpuinfo, this is
> > confused to user when turbo is enable, and aperf/mperf can be used to
> > show current cpu frequency after 7d5905dc14a
> > "(x86 / CPU: Always show current CPU frequency in /proc/cpuinfo)"
> > so guest should support aperf/mperf capability
> >
> > This patch implements aperf/mperf by three mode: none, software
> > emulation, and pass-through
> >
> > None: default mode, guest does not support aperf/mperf
> >
> > Software emulation: the period of aperf/mperf in guest mode are
> > accumulated as emulated value
> >
> > Pass-though: it is only suitable for KVM_HINTS_REALTIME, Because that
> > hint guarantees we have a 1:1 vCPU:CPU binding and guaranteed no
> > over-commit.
> >
> > And a per-VM capability is added to configure aperfmperf mode
> >
> 
> [...]
> 
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index
> > cd708b0b460a..c960dda4251b 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -122,6 +122,14 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)
> >   					   MSR_IA32_MISC_ENABLE_MWAIT);
> >   	}
> >
> > +	best = kvm_find_cpuid_entry(vcpu, 6, 0);
> > +	if (best) {
> > +		if (guest_has_aperfmperf(vcpu->kvm) &&
> > +			boot_cpu_has(X86_FEATURE_APERFMPERF))
> > +			best->ecx |= 1;
> > +		else
> > +			best->ecx &= ~1;
> > +	}
> 
> In my understanding, KVM allows userspace to set a CPUID feature bit for
> guest even if hardware doesn't support the feature.
> 
> So what makes X86_FEATURE_APERFMPERF different here? Is there any
> concern I miss?
> 
> -Xiaoyao

Whether software emulation for aperf/mperf or pass-through depends on host cpu aperf/mperf feature.
 
Software emulation: the period of aperf/mperf in guest mode are accumulated as emulated value

-Li