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

[Like Xu]

On 2020/6/5 12:23, Li,Rongqing wrote:
> > -----éäåä-----
> > åää: Xu, Like [mailto:like.xu@xxxxxxxxx]
> > åéæé: 2020å6æ5æ 10:32
> > æää: Li,Rongqing <lirongqing@xxxxxxxxx>
> > æé: linux-kernel@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; x86@xxxxxxxxxx;
> > hpa@xxxxxxxxx; bp@xxxxxxxxx; mingo@xxxxxxxxxx; tglx@xxxxxxxxxxxxx;
> > jmattson@xxxxxxxxxx; wanpengli@xxxxxxxxxxx; vkuznets@xxxxxxxxxx;
> > sean.j.christopherson@xxxxxxxxx; pbonzini@xxxxxxxxxx; xiaoyao.li@xxxxxxxxx;
> > wei.huang2@xxxxxxx
> > äé: 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 passthroughï
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