[PATCH] KVM: X86: Emulate APERF/MPERF to report actual VCPU frequency

[PATCH] KVM: X86: Emulate APERF/MPERF to report actual VCPU frequency

[Like Xu]

The aperf/mperf are used to report current CPU frequency after 7d5905dc14a
"x86 / CPU: Always show current CPU frequency in /proc/cpuinfo". But guest
kernel always reports a fixed VCPU frequency in the /proc/cpuinfo, which
may confuse users especially when turbo is enabled on the host.

Emulate guest APERF/MPERF capability based their values on the host.

Co-developed-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Reviewed-by: Chai Wen <chaiwen@baidu.com>
Reviewed-by: Jia Lina <jialina01@baidu.com>
Signed-off-by: Like Xu <like.xu@linux.intel.com>
---
 arch/x86/include/asm/kvm_host.h | 12 ++++++
 arch/x86/kvm/cpuid.c            |  8 +++-
 arch/x86/kvm/x86.c              | 76 ++++++++++++++++++++++++++++++++-
 3 files changed, 94 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index f852ee350beb..c48b9a0a086e 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -539,6 +539,16 @@ struct kvm_vcpu_hv_stimer {
 	bool msg_pending;
 };
 
+/* vCPU thermal and power context */
+struct kvm_vcpu_hwp {
+	/* Hardware Coordination Feedback Capability (Presence of APERF/MPERF) */
+	bool hw_coord_fb_cap;
+	/* MPERF increases with a fixed frequency */
+	u64 mperf;
+	/* APERF increases with the current/actual frequency */
+	u64 aperf;
+};
+
 /* Hyper-V synthetic interrupt controller (SynIC)*/
 struct kvm_vcpu_hv_synic {
 	u64 version;
@@ -829,6 +839,8 @@ struct kvm_vcpu_arch {
 
 	/* AMD MSRC001_0015 Hardware Configuration */
 	u64 msr_hwcr;
+
+	struct kvm_vcpu_hwp hwp;
 };
 
 struct kvm_lpage_info {
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 8a294f9747aa..7057809e7cfd 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -78,6 +78,11 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)
 			apic->lapic_timer.timer_mode_mask = 1 << 17;
 	}
 
+	best = kvm_find_cpuid_entry(vcpu, 0x6, 0);
+	if (best && best->function == 0x6 &&
+	    boot_cpu_has(X86_FEATURE_APERFMPERF) && (best->ecx & 0x1))
+		vcpu->arch.hwp.hw_coord_fb_cap = true;
+
 	best = kvm_find_cpuid_entry(vcpu, 7, 0);
 	if (best && boot_cpu_has(X86_FEATURE_PKU) && best->function == 0x7)
 		cpuid_entry_change(best, X86_FEATURE_OSPKE,
@@ -561,7 +566,8 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 	case 6: /* Thermal management */
 		entry->eax = 0x4; /* allow ARAT */
 		entry->ebx = 0;
-		entry->ecx = 0;
+		/* allow aperf/mperf to report the true VCPU frequency. */
+		entry->ecx = boot_cpu_has(X86_FEATURE_APERFMPERF) ? 0x1 : 0;
 		entry->edx = 0;
 		break;
 	/* function 7 has additional index. */
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 00c88c2f34e4..d220d9cc904a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3056,6 +3056,16 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 			return 1;
 		vcpu->arch.msr_misc_features_enables = data;
 		break;
+	case MSR_IA32_MPERF:
+		if (!msr_info->host_initiated && !vcpu->arch.hwp.hw_coord_fb_cap)
+			return 1;
+		vcpu->arch.hwp.mperf = 0;
+		return 0;
+	case MSR_IA32_APERF:
+		if (!msr_info->host_initiated && !vcpu->arch.hwp.hw_coord_fb_cap)
+			return 1;
+		vcpu->arch.hwp.aperf = 0;
+		return 0;
 	default:
 		if (msr && (msr == vcpu->kvm->arch.xen_hvm_config.msr))
 			return xen_hvm_config(vcpu, data);
@@ -3323,6 +3333,16 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	case MSR_K7_HWCR:
 		msr_info->data = vcpu->arch.msr_hwcr;
 		break;
+	case MSR_IA32_MPERF:
+		if (!msr_info->host_initiated && !vcpu->arch.hwp.hw_coord_fb_cap)
+			return 1;
+		msr_info->data = vcpu->arch.hwp.mperf;
+		break;
+	case MSR_IA32_APERF:
+		if (!msr_info->host_initiated && !vcpu->arch.hwp.hw_coord_fb_cap)
+			return 1;
+		msr_info->data = vcpu->arch.hwp.aperf;
+		break;
 	default:
 		if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
 			return kvm_pmu_get_msr(vcpu, msr_info);
@@ -8300,6 +8320,50 @@ void __kvm_request_immediate_exit(struct kvm_vcpu *vcpu)
 }
 EXPORT_SYMBOL_GPL(__kvm_request_immediate_exit);
 
+static inline void get_host_amperf(u64 *mperf, u64 *aperf)
+{
+	rdmsrl(MSR_IA32_MPERF, *mperf);
+	rdmsrl(MSR_IA32_APERF, *aperf);
+}
+
+static inline u64 get_amperf_delta(u64 enter, u64 exit)
+{
+	return (exit >= enter) ? (exit - enter) : (ULONG_MAX - enter + exit);
+}
+
+static inline void vcpu_update_amperf(struct kvm_vcpu *vcpu, u64 adelta, u64 mdelta)
+{
+	u64 aperf_left, mperf_left, delta, tmp;
+
+	aperf_left = ULONG_MAX - vcpu->arch.hwp.aperf;
+	mperf_left = ULONG_MAX - vcpu->arch.hwp.mperf;
+
+	/* fast path when neither MSR overflows */
+	if (adelta <= aperf_left && mdelta <= mperf_left) {
+		vcpu->arch.hwp.aperf += adelta;
+		vcpu->arch.hwp.mperf += mdelta;
+		return;
+	}
+
+	/* when either MSR overflows, both MSRs are reset to zero and continue to increment. */
+	delta = min(adelta, mdelta);
+	if (delta > aperf_left || delta > mperf_left) {
+		tmp = max(vcpu->arch.hwp.aperf, vcpu->arch.hwp.mperf);
+		tmp = delta - (ULONG_MAX - tmp) - 1;
+		vcpu->arch.hwp.aperf = tmp + adelta - delta;
+		vcpu->arch.hwp.mperf = tmp + mdelta - delta;
+		return;
+	}
+
+	if (mdelta > adelta && mdelta > aperf_left) {
+		vcpu->arch.hwp.mperf = mdelta - mperf_left - 1;
+		vcpu->arch.hwp.aperf = 0;
+	} else {
+		vcpu->arch.hwp.mperf = 0;
+		vcpu->arch.hwp.aperf = adelta - aperf_left - 1;
+	}
+}
+
 /*
  * Returns 1 to let vcpu_run() continue the guest execution loop without
  * exiting to the userspace.  Otherwise, the value will be returned to the
@@ -8312,7 +8376,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		dm_request_for_irq_injection(vcpu) &&
 		kvm_cpu_accept_dm_intr(vcpu);
 	fastpath_t exit_fastpath;
-
+	u64 enter_mperf = 0, enter_aperf = 0, exit_mperf = 0, exit_aperf = 0;
 	bool req_immediate_exit = false;
 
 	if (kvm_request_pending(vcpu)) {
@@ -8516,8 +8580,17 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
 	}
 
+	if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap))
+		get_host_amperf(&enter_mperf, &enter_aperf);
+
 	exit_fastpath = kvm_x86_ops.run(vcpu);
 
+	if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap)) {
+		get_host_amperf(&exit_mperf, &exit_aperf);
+		vcpu_update_amperf(vcpu, get_amperf_delta(enter_aperf, exit_aperf),
+			get_amperf_delta(enter_mperf, exit_mperf));
+	}
+
 	/*
 	 * Do this here before restoring debug registers on the host.  And
 	 * since we do this before handling the vmexit, a DR access vmexit
@@ -9482,6 +9555,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
 
 	vcpu->arch.pending_external_vector = -1;
 	vcpu->arch.preempted_in_kernel = false;
+	vcpu->arch.hwp.hw_coord_fb_cap = false;
 
 	kvm_hv_vcpu_init(vcpu);
 
-- 
2.21.3

Re: [PATCH] KVM: X86: Emulate APERF/MPERF to report actual VCPU frequency

[Sean Christopherson]

On Tue, Jun 23, 2020 at 02:35:30PM +0800, Like Xu wrote:
> The aperf/mperf are used to report current CPU frequency after 7d5905dc14a
> "x86 / CPU: Always show current CPU frequency in /proc/cpuinfo". But guest
> kernel always reports a fixed VCPU frequency in the /proc/cpuinfo, which
> may confuse users especially when turbo is enabled on the host.
> 
> Emulate guest APERF/MPERF capability based their values on the host.
> 
> Co-developed-by: Li RongQing <lirongqing@baidu.com>
> Signed-off-by: Li RongQing <lirongqing@baidu.com>
> Reviewed-by: Chai Wen <chaiwen@baidu.com>
> Reviewed-by: Jia Lina <jialina01@baidu.com>
> Signed-off-by: Like Xu <like.xu@linux.intel.com>
> ---

...

> @@ -8312,7 +8376,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  		dm_request_for_irq_injection(vcpu) &&
>  		kvm_cpu_accept_dm_intr(vcpu);
>  	fastpath_t exit_fastpath;
> -
> +	u64 enter_mperf = 0, enter_aperf = 0, exit_mperf = 0, exit_aperf = 0;
>  	bool req_immediate_exit = false;
>  
>  	if (kvm_request_pending(vcpu)) {
> @@ -8516,8 +8580,17 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  		vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
>  	}
>  
> +	if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap))
> +		get_host_amperf(&enter_mperf, &enter_aperf);
> +
>  	exit_fastpath = kvm_x86_ops.run(vcpu);
>  
> +	if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap)) {
> +		get_host_amperf(&exit_mperf, &exit_aperf);
> +		vcpu_update_amperf(vcpu, get_amperf_delta(enter_aperf, exit_aperf),
> +			get_amperf_delta(enter_mperf, exit_mperf));
> +	}
> +

Is there an alternative approach that doesn't require 4 RDMSRs on every VMX
round trip?  That's literally more expensive than VM-Enter + VM-Exit
combined.

E.g. what about adding KVM_X86_DISABLE_EXITS_APERF_MPERF and exposing the
MSRs for read when that capability is enabled?

Re: [PATCH] KVM: X86: Emulate APERF/MPERF to report actual VCPU frequency

[Jim Mattson]

On Tue, Jun 23, 2020 at 11:29 AM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Tue, Jun 23, 2020 at 02:35:30PM +0800, Like Xu wrote:
> > The aperf/mperf are used to report current CPU frequency after 7d5905dc14a
> > "x86 / CPU: Always show current CPU frequency in /proc/cpuinfo". But guest
> > kernel always reports a fixed VCPU frequency in the /proc/cpuinfo, which
> > may confuse users especially when turbo is enabled on the host.
> >
> > Emulate guest APERF/MPERF capability based their values on the host.
> >
> > Co-developed-by: Li RongQing <lirongqing@baidu.com>
> > Signed-off-by: Li RongQing <lirongqing@baidu.com>
> > Reviewed-by: Chai Wen <chaiwen@baidu.com>
> > Reviewed-by: Jia Lina <jialina01@baidu.com>
> > Signed-off-by: Like Xu <like.xu@linux.intel.com>
> > ---
>
> ...
>
> > @@ -8312,7 +8376,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >               dm_request_for_irq_injection(vcpu) &&
> >               kvm_cpu_accept_dm_intr(vcpu);
> >       fastpath_t exit_fastpath;
> > -
> > +     u64 enter_mperf = 0, enter_aperf = 0, exit_mperf = 0, exit_aperf = 0;
> >       bool req_immediate_exit = false;
> >
> >       if (kvm_request_pending(vcpu)) {
> > @@ -8516,8 +8580,17 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >               vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
> >       }
> >
> > +     if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap))
> > +             get_host_amperf(&enter_mperf, &enter_aperf);
> > +
> >       exit_fastpath = kvm_x86_ops.run(vcpu);
> >
> > +     if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap)) {
> > +             get_host_amperf(&exit_mperf, &exit_aperf);
> > +             vcpu_update_amperf(vcpu, get_amperf_delta(enter_aperf, exit_aperf),
> > +                     get_amperf_delta(enter_mperf, exit_mperf));
> > +     }
> > +
>
> Is there an alternative approach that doesn't require 4 RDMSRs on every VMX
> round trip?  That's literally more expensive than VM-Enter + VM-Exit
> combined.
>
> E.g. what about adding KVM_X86_DISABLE_EXITS_APERF_MPERF and exposing the
> MSRs for read when that capability is enabled?

When would you load the hardware MSRs with the guest/host values?

Re: [PATCH] KVM: X86: Emulate APERF/MPERF to report actual VCPU frequency

[Sean Christopherson]

On Tue, Jun 23, 2020 at 11:39:16AM -0700, Jim Mattson wrote:
> On Tue, Jun 23, 2020 at 11:29 AM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
> >
> > On Tue, Jun 23, 2020 at 02:35:30PM +0800, Like Xu wrote:
> > > The aperf/mperf are used to report current CPU frequency after 7d5905dc14a
> > > "x86 / CPU: Always show current CPU frequency in /proc/cpuinfo". But guest
> > > kernel always reports a fixed VCPU frequency in the /proc/cpuinfo, which
> > > may confuse users especially when turbo is enabled on the host.
> > >
> > > Emulate guest APERF/MPERF capability based their values on the host.
> > >
> > > Co-developed-by: Li RongQing <lirongqing@baidu.com>
> > > Signed-off-by: Li RongQing <lirongqing@baidu.com>
> > > Reviewed-by: Chai Wen <chaiwen@baidu.com>
> > > Reviewed-by: Jia Lina <jialina01@baidu.com>
> > > Signed-off-by: Like Xu <like.xu@linux.intel.com>
> > > ---
> >
> > ...
> >
> > > @@ -8312,7 +8376,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> > >               dm_request_for_irq_injection(vcpu) &&
> > >               kvm_cpu_accept_dm_intr(vcpu);
> > >       fastpath_t exit_fastpath;
> > > -
> > > +     u64 enter_mperf = 0, enter_aperf = 0, exit_mperf = 0, exit_aperf = 0;
> > >       bool req_immediate_exit = false;
> > >
> > >       if (kvm_request_pending(vcpu)) {
> > > @@ -8516,8 +8580,17 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> > >               vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
> > >       }
> > >
> > > +     if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap))
> > > +             get_host_amperf(&enter_mperf, &enter_aperf);
> > > +
> > >       exit_fastpath = kvm_x86_ops.run(vcpu);
> > >
> > > +     if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap)) {
> > > +             get_host_amperf(&exit_mperf, &exit_aperf);
> > > +             vcpu_update_amperf(vcpu, get_amperf_delta(enter_aperf, exit_aperf),
> > > +                     get_amperf_delta(enter_mperf, exit_mperf));
> > > +     }
> > > +
> >
> > Is there an alternative approach that doesn't require 4 RDMSRs on every VMX
> > round trip?  That's literally more expensive than VM-Enter + VM-Exit
> > combined.
> >
> > E.g. what about adding KVM_X86_DISABLE_EXITS_APERF_MPERF and exposing the
> > MSRs for read when that capability is enabled?
> 
> When would you load the hardware MSRs with the guest/host values?

Ugh, I was thinking the MSRs were read-only.

Doesn't this also interact with TSC scaling?

Re: [PATCH] KVM: X86: Emulate APERF/MPERF to report actual VCPU frequency

[Jim Mattson]

On Tue, Jun 23, 2020 at 12:05 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Tue, Jun 23, 2020 at 11:39:16AM -0700, Jim Mattson wrote:
> > On Tue, Jun 23, 2020 at 11:29 AM Sean Christopherson
> > <sean.j.christopherson@intel.com> wrote:
> > >
> > > On Tue, Jun 23, 2020 at 02:35:30PM +0800, Like Xu wrote:
> > > > The aperf/mperf are used to report current CPU frequency after 7d5905dc14a
> > > > "x86 / CPU: Always show current CPU frequency in /proc/cpuinfo". But guest
> > > > kernel always reports a fixed VCPU frequency in the /proc/cpuinfo, which
> > > > may confuse users especially when turbo is enabled on the host.
> > > >
> > > > Emulate guest APERF/MPERF capability based their values on the host.
> > > >
> > > > Co-developed-by: Li RongQing <lirongqing@baidu.com>
> > > > Signed-off-by: Li RongQing <lirongqing@baidu.com>
> > > > Reviewed-by: Chai Wen <chaiwen@baidu.com>
> > > > Reviewed-by: Jia Lina <jialina01@baidu.com>
> > > > Signed-off-by: Like Xu <like.xu@linux.intel.com>
> > > > ---
> > >
> > > ...
> > >
> > > > @@ -8312,7 +8376,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> > > >               dm_request_for_irq_injection(vcpu) &&
> > > >               kvm_cpu_accept_dm_intr(vcpu);
> > > >       fastpath_t exit_fastpath;
> > > > -
> > > > +     u64 enter_mperf = 0, enter_aperf = 0, exit_mperf = 0, exit_aperf = 0;
> > > >       bool req_immediate_exit = false;
> > > >
> > > >       if (kvm_request_pending(vcpu)) {
> > > > @@ -8516,8 +8580,17 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> > > >               vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
> > > >       }
> > > >
> > > > +     if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap))
> > > > +             get_host_amperf(&enter_mperf, &enter_aperf);
> > > > +
> > > >       exit_fastpath = kvm_x86_ops.run(vcpu);
> > > >
> > > > +     if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap)) {
> > > > +             get_host_amperf(&exit_mperf, &exit_aperf);
> > > > +             vcpu_update_amperf(vcpu, get_amperf_delta(enter_aperf, exit_aperf),
> > > > +                     get_amperf_delta(enter_mperf, exit_mperf));
> > > > +     }
> > > > +
> > >
> > > Is there an alternative approach that doesn't require 4 RDMSRs on every VMX
> > > round trip?  That's literally more expensive than VM-Enter + VM-Exit
> > > combined.
> > >
> > > E.g. what about adding KVM_X86_DISABLE_EXITS_APERF_MPERF and exposing the
> > > MSRs for read when that capability is enabled?
> >
> > When would you load the hardware MSRs with the guest/host values?
>
> Ugh, I was thinking the MSRs were read-only.

EVen if they were read-only, they should power on to zero, and they
will most likely not be zero when a guest powers on.

> Doesn't this also interact with TSC scaling?

Yes, it should!

Re: [PATCH] KVM: X86: Emulate APERF/MPERF to report actual VCPU frequency

[Like Xu]

On 24/6/2020 4:34 am, Jim Mattson wrote:
> On Tue, Jun 23, 2020 at 12:05 PM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
>>
>> On Tue, Jun 23, 2020 at 11:39:16AM -0700, Jim Mattson wrote:
>>> On Tue, Jun 23, 2020 at 11:29 AM Sean Christopherson
>>> <sean.j.christopherson@intel.com> wrote:
>>>>
>>>> On Tue, Jun 23, 2020 at 02:35:30PM +0800, Like Xu wrote:
>>>>> The aperf/mperf are used to report current CPU frequency after 7d5905dc14a
>>>>> "x86 / CPU: Always show current CPU frequency in /proc/cpuinfo". But guest
>>>>> kernel always reports a fixed VCPU frequency in the /proc/cpuinfo, which
>>>>> may confuse users especially when turbo is enabled on the host.
>>>>>
>>>>> Emulate guest APERF/MPERF capability based their values on the host.
>>>>>
>>>>> Co-developed-by: Li RongQing <lirongqing@baidu.com>
>>>>> Signed-off-by: Li RongQing <lirongqing@baidu.com>
>>>>> Reviewed-by: Chai Wen <chaiwen@baidu.com>
>>>>> Reviewed-by: Jia Lina <jialina01@baidu.com>
>>>>> Signed-off-by: Like Xu <like.xu@linux.intel.com>
>>>>> ---
>>>>
>>>> ...
>>>>
>>>>> @@ -8312,7 +8376,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>>>>                dm_request_for_irq_injection(vcpu) &&
>>>>>                kvm_cpu_accept_dm_intr(vcpu);
>>>>>        fastpath_t exit_fastpath;
>>>>> -
>>>>> +     u64 enter_mperf = 0, enter_aperf = 0, exit_mperf = 0, exit_aperf = 0;
>>>>>        bool req_immediate_exit = false;
>>>>>
>>>>>        if (kvm_request_pending(vcpu)) {
>>>>> @@ -8516,8 +8580,17 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>>>>                vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
>>>>>        }
>>>>>
>>>>> +     if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap))
>>>>> +             get_host_amperf(&enter_mperf, &enter_aperf);
>>>>> +
>>>>>        exit_fastpath = kvm_x86_ops.run(vcpu);
>>>>>
>>>>> +     if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap)) {
>>>>> +             get_host_amperf(&exit_mperf, &exit_aperf);
>>>>> +             vcpu_update_amperf(vcpu, get_amperf_delta(enter_aperf, exit_aperf),
>>>>> +                     get_amperf_delta(enter_mperf, exit_mperf));
>>>>> +     }
>>>>> +
>>>>
>>>> Is there an alternative approach that doesn't require 4 RDMSRs on every VMX
>>>> round trip?  That's literally more expensive than VM-Enter + VM-Exit
>>>> combined.

It looks like we have quite a few users who are expecting this feature in 
different scenarios.

I will add a fast path for RO usage and a slow path if the guest tries to change 
the AMPERF values.

>>>>
>>>> E.g. what about adding KVM_X86_DISABLE_EXITS_APERF_MPERF and exposing the
>>>> MSRs for read when that capability is enabled?
>>>
>>> When would you load the hardware MSRs with the guest/host values?
>>
>> Ugh, I was thinking the MSRs were read-only.
> 
> EVen if they were read-only, they should power on to zero, and they
> will most likely not be zero when a guest powers on.

Can we assume that "not zero when the guest is on" will not harm any guests ?

> 
>> Doesn't this also interact with TSC scaling?
> 
> Yes, it should!

We have too much of a historical burden on TSC emulations.

For practical reasons, what if we only expose the AMPERF cap
if the host/guest has both CONSTANT_TSC and NONSTOP_TSC ?

One more design concern, I wonder if it is *safe* for the guest to
read amperf on pCPU[x] the first time and on pCPU[y] the next time.

Any input ?

Thanks,
Like Xu