On 2020/6/5 12:23, Li,Rongqing wrote:
-----邮件原件-----
发件人: Xu, Like [mailto:like.xu@xxxxxxxxx]
发送时间: 2020年6月5日 10:32
收件人: Li,Rongqing <lirongqing@xxxxxxxxx>
抄送: linux-kernel@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; x86@xxxxxxxxxx;
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jmattson@xxxxxxxxxx; wanpengli@xxxxxxxxxxx; vkuznets@xxxxxxxxxx;
sean.j.christopherson@xxxxxxxxx; pbonzini@xxxxxxxxxx; xiaoyao.li@xxxxxxxxx;
wei.huang2@xxxxxxx
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
