Re: [PATCH v2] kvm: x86: emulate monitor and mwait instructions as nop

Michael S. Tsirkin
2014-06-02T20:41:03Z

On Mon, Jun 02, 2014 at 10:35:56PM +0200, Alexander Graf wrote:
>
>
> > Am 02.06.2014 um 22:20 schrieb "Michael S. Tsirkin" <mst@redhat.com>:
> >
> >> On Mon, Jun 02, 2014 at 09:48:19PM +0200, Alexander Graf wrote:
> >>
> >>
> >>>> Am 02.06.2014 um 21:25 schrieb "Gabriel L. Somlo" <gsomlo@gmail.com>:
> >>>>
> >>>> On Wed, May 07, 2014 at 04:52:13PM -0400, Gabriel L. Somlo wrote:
> >>>> Treat monitor and mwait instructions as nop, which is architecturally
> >>>> correct (but inefficient) behavior. We do this to prevent misbehaving
> >>>> guests (e.g. OS X <= 10.7) from crashing after they fail to check for
> >>>> monitor/mwait availability via cpuid.
> >>>>
> >>>> Since mwait-based idle loops relying on these nop-emulated instructions
> >>>> would keep the host CPU pegged at 100%, do NOT advertise their presence
> >>>> via cpuid, to prevent compliant guests from using them inadvertently.
> >>>>
> >>>> Signed-off-by: Gabriel L. Somlo <somlo@cmu.edu>
> >>>> ---
> >>>>
> >>>> New in v2: remove invalid_op handler functions which were only used to
> >>>> handle exits caused by monitor and mwait
> >>>>
> >>>>>> On Wed, May 07, 2014 at 08:31:27PM +0200, Alexander Graf wrote:
> >>>>>> On 05/07/2014 08:15 PM, Michael S. Tsirkin wrote:
> >>>>>> If we really want to be paranoid and worry about guests
> >>>>>> that use this strange way to trigger invalid opcode,
> >>>>>> we can make it possible for userspace to enable/disable
> >>>>>> this hack, and teach qemu to set it.
> >>>>>>
> >>>>>> That would make it even safer than it was.
> >>>>>>
> >>>>>> Not sure it's worth it, just a thought.
> >>>>>
> >>>>> Since we don't trap on non-exposed other instructions (new SSE and
> >>>>> whatdoiknow) I don't think it's really bad to just expose
> >>>>> MONITOR/MWAIT as nops.
> >>>
> >>> Would it make sense to make this a module parameter,
> >>> (e.g., "int emulate_mwait") ?
> >>>
> >>> Default would be 0 (no emulation). 1 would mean "emulate as nop", and
> >>> if anyone ever figures out how to do proper page-locking based
> >>> emulation we could use 2 to enable that, etc. ?
> >>>
> >>> Not sure we'd want qemu to enable/disable it automatically, though...
> >>>
> >>> What do you all think ?
> >>
> >> I don't like module parameters - they're system global and there's a good chance you want to run non-osx in parallel ;).
> >>
> >> I'd either link this to the cpuid bits or enable it forcefully through ENABLE_CAP per vcpu.
> >>
> >> Alex
> >
> > Point is that.
> > Paolo here thinks it's safe to just make it a NOP unconditionally.
> > so module parameter would be there as a debugging tool:
> > as a means for users to test with old kvm behaviour if they see breakage.
> > Which we don't expect, so no need to waste cycles creating a pretty
> > interface for it.
>
> Both interfaces already exist, so where's the problem?

Hmm sorry which interfaces for enabling mwait nop emulation exist?

> I'm fine with making it always nop too though.
>
> Gabriel was asking how to make it switchable - and the only thing I'd nak is a module parameter because it's not useful.
>
>
> Alex
>

--
MST