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

Radim Krčmář
2017-03-28T14:28:37Z

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 ...

And before we disable MWAIT exiting by default, we also have to
understand the old OS X on core 2 bug from Gabriel.