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

[Alexander Graf]

On 04/04/2017 02:39 PM, Radim KrÄmÃÅ wrote:
> 2017-04-03 12:04+0200, Alexander Graf:
> > On 03/29/2017 02:11 PM, Radim KrÄmÃÅ wrote:
> > > 2017-03-28 13:35-0700, Jim Mattson:
> > > > On Tue, Mar 28, 2017 at 7:28 AM, Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx> wrote:
> > > > > 2017-03-27 15:34+0200, Alexander Graf:
> > > > > > On 15/03/2017 22:22, 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.
> > > > > > So imagine we had proper MWAIT emulation capabilities based on page faults.
> > > > > > In that case, we could do something as fancy as
> > > > > >
> > > > > > Treat MWAIT as pass-through by default
> > > > > >
> > > > > > Have a per-vcpu monitor timer 10 times a second in the background that
> > > > > > checks which instruction we're in
> > > > > >
> > > > > > If we're in mwait for the last - say - 1 second, switch to emulated MWAIT,
> > > > > > if $IP was in non-mwait within that time, reset counter.
> > > > > Or we could reuse external interrupts for sampling.  Exits trigerred by
> > > > > them would check for current instruction (probably would be best to
> > > > > limit just to timer tick) and a sufficient ratio (> 0?) of other exits
> > > > > would imply that MWAIT is not used.
> > > > >
> > > > > > Or instead maybe just reuse the adapter hlt logic?
> > > > > Emulated MWAIT is very similar to emulated HLT, so reusing the logic
> > > > > makes sense.  We would just add new wakeup methods.
> > > > >
> > > > > > Either way, with that we should be able to get super low latency IPIs
> > > > > > running while still maintaining some sanity on systems which don't have
> > > > > > dedicated CPUs for workloads.
> > > > > >
> > > > > > And we wouldn't need guest modifications, which is a great plus. So older
> > > > > > guests (and Windows?) could benefit from mwait as well.
> > > > > There is no need guest modifications -- it could be exposed as standard
> > > > > MWAIT feature to the guest, with responsibilities for guest/host-impact
> > > > > on the user.
> > > > >
> > > > > I think that the page-fault based MWAIT would require paravirt if it
> > > > > should be enabled by default, because of performance concerns:
> > > > > Enabling write protection on a page needs a VM exit on all other VCPUs
> > > > > when beginning monitoring (to reload page permissions and prevent missed
> > > > > writes).
> > > > > We'd want to keep trapping writes to the page all the time because
> > > > > toggling is slow, but this could regress performance for an OS that has
> > > > > other data accessed by other VCPUs in that page.
> > > > > No current interface can tell the guest that it should reserve the whole
> > > > > page instead of what CPUID[5] says and that writes to the monitored page
> > > > > are not "cheap", but can trigger a VM exit ...
> > > > CPUID.05H:EBX is supposed to address the false sharing issue. IIRC,
> > > > VMware Fusion reports 64 in CPUID.05H:EAX and 4096 in CPUID.05H:EBX
> > > > when running Mac OS X guests. Per Intel's SDM volume 3, section
> > > > 8.10.5, "To avoid false wake-ups; use the largest monitor line size to
> > > > pad the data structure used to monitor writes. Software must make sure
> > > > that beyond the data structure, no unrelated data variable exists in
> > > > the triggering area for MWAIT. A pad may be needed to avoid this
> > > > situation." Unfortunately, most operating systems do not follow this
> > > > advice.
> > > Right, EBX provides what we need to expose that the whole page is
> > > monitored, thanks!
> > So coming back to the original patch, is there anything that should keep us
> > from exposing MWAIT straight into the guest at all times?
> Just minor issues:
>   * OS X on Core 2 fails for unknown reason if we disable the instruction
>     trapping, which is an argument against doing it by default

So for that we should try and see if changing the exposed CPUID MWAIT 
leaf helps. Currently we return 0/0 which is pretty bogus and might be 
the reason OSX fails.

>   * idling guests would consume host CPU, which is a significant change
>     in behavior and shouldn't be done without userspace's involvement

That's the same as today, as idling guests with MWAIT would also today 
end up in a NOP emulated loop.

Please bear in mind that I do not advocate to expose the MWAIT CPUID 
flag. This is only for the instruction trap.

> I think the best compromise is to add a capability for the MWAIT VM-exit
> controls and let userspace expose MWAIT if it wishes to.
> Will send a patch.


Please see my patch to force enable CPUID bits ;).



Alex