答复: 答复: [PATCH][v6] KVM: X86: support APERF/MPERF registers

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

> -----邮件原件-----
> 发件人: Like Xu [mailto:like.xu@linux.intel.com]
> 发送时间: 2020年6月5日 13:29
> 收件人: Li,Rongqing <lirongqing@baidu.com>; like.xu@intel.com
> 抄送: 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; xiaoyao.li@intel.com;
> wei.huang2@amd.com
> 主题: Re: 答复: [PATCH][v6] KVM: X86: support APERF/MPERF registers
> 
> On 2020/6/5 12:23, Li,Rongqing wrote:
> >
> >
> >> -----邮件原件-----
> >> 发件人: Xu, Like [mailto:like.xu@intel.com]
> >> 发送时间: 2020年6月5日 10:32
> >> 收件人: Li,Rongqing <lirongqing@baidu.com>
> >> 抄送: 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; xiaoyao.li@intel.com; wei.huang2@amd.com
> >> 主题: Re: [PATCH][v6] KVM: X86: support APERF/MPERF registers
> >>
> >> Hi RongQing,
> >>
> >> On 2020/6/5 9:44, 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
> >> s/None/Note
> >>>
> >>> 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.
> >> The flag "KVM_HINTS_REALTIME 0" (in the
> >> Documentation/virt/kvm/cpuid.rst) is claimed as "guest checks this
> >> feature bit to determine that vCPUs are never preempted for an unlimited
> time allowing optimizations".
> >>
> >> I couldn't see its relationship with "1:1 vCPU: pCPU binding".
> >> The patch doesn't check this flag as well for your pass-through purpose.
> >>
> >> Thanks,
> >> Like Xu
> >
> >
> > I think this is user space jobs to bind HINT_REALTIME and mperf passthrough,
> KVM just do what userspace wants.
> >
> 
> That's fine for user space to bind HINT_REALTIME and mperf passthrough，
> But I was asking why HINT_REALTIME means "1:1 vCPU: pCPU binding".
> 
> As you said, "Pass-though: it is only suitable for KVM_HINTS_REALTIME", which
> means, KVM needs to make sure the kvm->arch.aperfmperf_mode value could
> "only" be set to KVM_APERFMPERF_PT when the check
> kvm_para_has_hint(KVM_HINTS_REALTIME) is passed.
> 

pining vcpu can ensure that guest get correct mperf/aperf, but a user
has the choice to not pin, at that condition, do not think it is bug, this wants to say

> Specifically, the KVM_HINTS_REALTIME is a per-kvm capability while the
> kvm_aperfmperf_mode is a per-vm capability. It's unresolved.
> 

Do you have any solution?

-Rongqing

> KVM doesn't always do what userspace wants especially you're trying to
> expose some features about power and thermal management in the
> virtualization context.
> 
> > and this gives user space a possibility, guest has passthrough
> > mperfaperf without HINT_REALTIME, guest can get coarse cpu frequency
> > without performance effect if guest can endure error frequency
> > occasionally
> >
> 
> 
> >
> > -Li
> >