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

[Like Xu <like.xu@linux.intel.com>]

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
>