linux-kernel - Re: [PATCH][v2] kvm: x86: emulate APERF/MPERF registers

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Message-ID: <20200429085440.GG13592@hirez.programming.kicks-ass.net>
Date:   Wed, 29 Apr 2020 10:54:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Li RongQing <lirongqing@...du.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org, x86@...nel.org,
        hpa@...or.com, bp@...en8.de, mingo@...hat.com, tglx@...utronix.de,
        joro@...tes.org, jmattson@...gle.com, wanpengli@...cent.com,
        vkuznets@...hat.com, sean.j.christopherson@...el.com,
        pbonzini@...hat.com
Subject: Re: [PATCH][v2] kvm: x86: emulate APERF/MPERF registers

On Wed, Apr 29, 2020 at 01:46:36PM +0800, 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 we should emulate aperf mperf to achieve it
> 
> the period of aperf/mperf in guest mode are accumulated as
> emulated value, and add per-VM knod to enable emulate mperfaperf
> 
> diff v1:
> 1. support AMD
> 2. support per-vm capability to enable

Would it make sense to provide a pass-through APERF/MPERF for
KVM_HINTS_REALTIME ? Because that hint guarantees we have a 1:1 vCPU:CPU
binding and guaranteed no over-commit.