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

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Date:   Tue, 23 Jun 2020 11:29:10 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Like Xu <like.xu@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, wei.huang2@....com,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Li RongQing <lirongqing@...du.com>,
        Chai Wen <chaiwen@...du.com>, Jia Lina <jialina01@...du.com>
Subject: Re: [PATCH] KVM: X86: Emulate APERF/MPERF to report actual VCPU
 frequency

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@...du.com>
> Signed-off-by: Li RongQing <lirongqing@...du.com>
> Reviewed-by: Chai Wen <chaiwen@...du.com>
> Reviewed-by: Jia Lina <jialina01@...du.com>
> Signed-off-by: Like Xu <like.xu@...ux.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?