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

Michael S. Tsirkin
2017-03-15T23:41:28Z

On Wed, Mar 15, 2017 at 07:35:34PM -0400, Gabriel L. Somlo wrote:
> On Wed, Mar 15, 2017 at 11:22:18PM +0200, Michael S. Tsirkin wrote:
> > Guests running Mac OS 5, 6, and 7 (Leopard through Lion) have a problem:
> > unless explicitly provided with kernel command line argument
> > "idlehalt=0" they'd implicitly assume MONITOR and MWAIT availability,
> > without checking CPUID.
> >
> > We currently emulate that as a NOP but on VMX we can do better: let
> > guest stop the CPU until timer, IPI or memory change. CPU will be busy
> > but that isn't any worse than a NOP emulation.
> >
> > Note that mwait within guests is not the same as on real hardware
> > because halt causes an exit while mwait doesn't. For this reason it
> > might not be a good idea to use the regular MWAIT flag in CPUID to
> > signal this capability. Add a flag in the hypervisor leaf instead.
> >
> > Additionally, we add a capability for QEMU - e.g. if it knows there's an
> > isolated CPU dedicated for the VCPU it can set the standard MWAIT flag
> > to improve guest behaviour.
>
> Same behavior (on the mac pro 1,1 running F22 with custom-compiled
> kernel from kvm git master, plus this patch on top).
>
> The OS X 10.7 kernel hangs (or at least progresses extremely slowly)
> on boot, does not bring up guest graphical interface within the first
> 10 minutes that I waited for it. That, in contrast with the default
> nop-based emulation where the guest comes up within 30 seconds.

Thanks a lot, meanwhile I'll try to write a unit-test and experiment
with various behaviours.

> I will run another round of tests on a newer Mac (4-year-old macbook
> air) and report back tomorrow.
>
> Going off on a tangent, why would encouraging otherwise well-behaved
> guests (like linux ones, for example) to use MWAIT be desirable to
> begin with ? Is it a matter of minimizing the overhead associated with
> exiting and re-entering L1 ? Because if so, AFAIR staying inside L1 and
> running guest-mode MWAIT in a tight loop will actually waste the host
> CPU without the opportunity to yield to some other L0 thread. Sorry if
> I fell into the middle of an ongoing conversation on this and missed
> most of the relevant context, in which case please feel free to ignore
> me... :)
>
> Thanks,
> --G

It's just some experiments I'm running, I'm not ready to describe it
yet. I thought this part might be useful to at least some guests, so
trying to upstream it right now.

--
MST