Re: çå: [PATCH][v5] KVM: X86: support APERF/MPERF registers

From: Xiaoyao Li
Date: Sat May 30 2020 - 22:35:40 EST

On 5/31/2020 10:08 AM, Li,Rongqing wrote:

åää: Xiaoyao Li []
åéæé: 2020å5æ30æ 18:40
æää: Li,Rongqing <lirongqing@xxxxxxxxx>; 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;
äé: Re: [PATCH][v5] KVM: X86: support APERF/MPERF registers

On 5/30/2020 12:35 PM, 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

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

And a per-VM capability is added to configure aperfmperf mode


diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index
cd708b0b460a..c960dda4251b 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -122,6 +122,14 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)

+ best = kvm_find_cpuid_entry(vcpu, 6, 0);
+ if (best) {
+ if (guest_has_aperfmperf(vcpu->kvm) &&
+ boot_cpu_has(X86_FEATURE_APERFMPERF))
+ best->ecx |= 1;
+ else
+ best->ecx &= ~1;
+ }

In my understanding, KVM allows userspace to set a CPUID feature bit for
guest even if hardware doesn't support the feature.

So what makes X86_FEATURE_APERFMPERF different here? Is there any
concern I miss?


Whether software emulation for aperf/mperf or pass-through depends on host cpu aperf/mperf feature.
Software emulation: the period of aperf/mperf in guest mode are accumulated as emulated value

I know it that you want to ensure the correctness of exposure of aperf/mperf.

But there are so many features other than aperf/mperf that KVM reports the supported settings of them through KVM_GET_SUPPORTED_CPUID, but doesn't check nor force the correctness of userspace input. i.e., KVM allows userspace to set bogus CPUID settings as long as it doesn't break KVM (host kernel).

Indeed, bogus CPUID settings more than likely breaks the guest. But it's not KVM's fault. KVM just do what userspace wants.

IMO, If we really want to ensure the correctness of userspace provided CPUID settings, we need to return ERROR to userspace instead of fixing it siliently.

- Xiaoyao