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

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Date:   Mon, 27 Apr 2020 10:30:22 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     "Li,Rongqing" <lirongqing@...du.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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 Sat, Apr 25, 2020 at 8:24 PM Li,Rongqing <lirongqing@...du.com> wrote:
>
>
>
> > -----邮件原件-----
> > 发件人: Paolo Bonzini [mailto:pbonzini@...hat.com]
> > 发送时间: 2020年4月25日 0:30
> > 收件人: Jim Mattson <jmattson@...gle.com>; Sean Christopherson
> > <sean.j.christopherson@...el.com>
> > 抄送: 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>
> > 主题: 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
>
> I will add a kvm parameter to consider whether enable MPERF/APERF emulations, and make default value to false

Wouldn't it be better to add a per-VM capability to enable this feature?