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

[Wanpeng Li]

Cc Peterz,
2017-03-10 9:12 GMT+08:00 Michael S. Tsirkin <mst@xxxxxxxxxx>:
> On Thu, Mar 09, 2017 at 07:51:27PM -0500, Gabriel L. Somlo wrote:
>> On Fri, Mar 10, 2017 at 12:29:31AM +0200, Michael S. Tsirkin wrote:
>> > Some guests call mwait without checking the cpu flags.  We currently
>> > emulate that as a NOP but on VMX we can do better: let guest stop the
>> > CPU until timer or IPI.  CPU will be busy but that isn't any worse than
>> > a NOP emulation.
>>
>> Are you getting an IPI if another VCPU writes to the MONITOR-ed memory
>> location?
>
> In my testing yes.

Why there is still an IPI if monitor/mwait is used in guest?

>
>> If not, you'd be waking up too late and fail to meet the
>> specified behavior of the MONITOR/MWAIT instruction pair.
>>
>> > Note that mwait within guests is not the same as on real hardware
>> > because you must halt if you want to go deep into sleep.  Thus it isn't
>> > a good idea to use the regular MWAIT flag in CPUID for that.  Add a flag
>> > in the hypervisor leaf instead.
>>
>> Is it a good idea to advertise MWAIT capability to guests?
>
> I think it isn't so this patch does not do it.
>
>> The
>> misbehaving ones will call it willy-nilly, true, but aren't compliant
>> ones better off falling back to some alternative method (typically
>> using a HLT-based idle loop instead of a MONITOR/MWAIT based one) ?
>>
>> Thanks,
>> --Gabriel
>>

[...]

>> > @@ -594,6 +594,9 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>> >             if (sched_info_on())
>> >                     entry->eax |= (1 << KVM_FEATURE_STEAL_TIME);
>> >
>> > +           if (this_cpu_has(X86_FEATURE_MWAIT))
>> > +                   entry->eax = (1 << KVM_FEATURE_MWAIT);

s/"="/"|=", otherwise you almost kill other features.

Regards,
Wanpeng Li