Messages in this thread | | | Subject | Re: 答复: [PATCH][v6] KVM: X86: support APERF/MPERF registers | From | Like Xu <> | Date | Fri, 5 Jun 2020 13:28:30 +0800 |
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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.
Specifically, the KVM_HINTS_REALTIME is a per-kvm capability while the kvm_aperfmperf_mode is a per-vm capability. It's unresolved.
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 >
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