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

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Message-ID: <ce51d5f9-aa7b-233b-883d-802d9b00e090@redhat.com>
Date:   Fri, 24 Apr 2020 18:30:18 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Li RongQing <lirongqing@...du.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joerg Roedel <joro@...tes.org>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH] [RFC] kvm: x86: emulate APERF/MPERF registers

On 24/04/20 18:25, Jim Mattson wrote:
>> Assuming we're going forward with this, at an absolute minimum the RDMSRs
>> need to be wrapped with checks on host _and_ guest support for the emulated
>> behavior.  Given the significant overhead, this might even be something
>> that should require an extra opt-in from userspace to enable.
> 
> I would like to see performance data before enabling this unconditionally.

I wouldn't want this to be enabled unconditionally anyway, because you
need to take into account live migration to and from processors that do
not have APERF/MPERF support.

Paolo