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

Gabriel L. Somlo
2017-03-22T14:10:05Z

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@gmail.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.