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

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Date:   Fri, 24 Apr 2020 09:25:06 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     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>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] [RFC] kvm: x86: emulate APERF/MPERF registers

On Fri, Apr 24, 2020 at 7:46 AM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Fri, Apr 24, 2020 at 12:01:43PM +0200, Peter Zijlstra wrote:
> > On Fri, Apr 24, 2020 at 01:08:55PM +0800, Li RongQing wrote:

> This requires four RDMSRs per VMX transition.  Doing that unconditionally
> will drastically impact performance.  Not to mention that reading the MSRs
> without checking for host support will generate #GPs and WARNs on hardware
> without APERFMPERF.
>
> 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.