linux-kernel - Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests

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Date:   Wed, 22 Mar 2017 10:10:05 -0400
From:   "Gabriel L. Somlo" <gsomlo@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Nadav Amit <nadav.amit@...il.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Joerg Roedel <joro@...tes.org>, KVM list <kvm@...r.kernel.org>,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests

On Wed, Mar 22, 2017 at 03:35:18PM +0200, Michael S. Tsirkin wrote:
> On Tue, Mar 21, 2017 at 05:02:25PM -0700, Nadav Amit wrote:
> > 
> > > On Mar 21, 2017, at 3:51 PM, Gabriel Somlo <gsomlo@...il.com> wrote:
> > > 
> > > And I get the exact same results on the MacBookAir4,2 (which exhibits
> > > no freezing or extreme sluggishness when running OS X 10.7 smp with
> > > Michael's KVM MWAIT-in-L1 patch)...
> > 
> > Sorry for my confusion. I didn’t read the entire thread and thought that
> > the problem is spurious wake-ups.
> > 
> > Since that is not the case, I would just suggest two things that you can
> > freely ignore:
> > 
> > 1. According to the SDM, when an interrupt is delivered, the interrupt
> > is only delivered on the following instruction, so you may consider
> > skipping the MWAIT first.
> > 
> > 2. Perhaps the CPU changes for some reason GUEST_ACTIVITY_STATE (which
> > is not according to the SDM).
> > 
> > That is it. No more BS from me.
> > 
> > Nadav
> 
> Intersting. I found this errata:
> A REP STOS/MOVS to a MONITOR/MWAIT Address Range May Prevent Triggering of
> the Monitoring Hardware

Any way to tell if they mean that for L0, or L>=1, or all of them?

> Could the macbook CPU be affected?

I ran a grep on the log file I collected when disassembling
AppleIntelCPUPowerManagement.kext (where the MWAIT-based idle
thread lives) a few days ago, and didn't find any "rep stos" or
"rep movs" instances.