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

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Message-ID: <201824a4-6b0f-9061-ec21-26d71fa11bc4@redhat.com>
Date:   Wed, 29 Apr 2020 12:21:25 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        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
Subject: Re: [PATCH][v2] kvm: x86: emulate APERF/MPERF registers

On 29/04/20 10:54, Peter Zijlstra wrote:
> 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.
> 

Yes but that's up to userspace.

Paolo