Re: [PATCH v6] kvm: better MWAIT emulation for guests

[Alexander Graf <agraf@suse.de>]

On 21.04.17 12:02, Paolo Bonzini wrote:
>
>
> On 12/04/2017 18:29, Michael S. Tsirkin wrote:
>> I don't really agree we do not need the PV flag. mwait on kvm is
>> different from mwait on bare metal in that you are heavily penalized by
>> scheduler for polling unless you configure the host just so.
>> HLT lets you give up the host CPU if you know you won't need
>> it for a long time.
>>
>> So while many people can get by with monitor cpuid (those that isolate
>> host CPUs) and it's a valuable option to have, I think a PV flag is also
>> a valuable option and can be set for more configurations.
>>
>> Guest has an idle driver calling mwait on short waits and halt on longer
>> ones.  I'm in fact testing an idle driver using such a PV flag and will
>> post when ready (after vacation ~3 weeks from now probably).
>
> For now I think I'm removing the PV flag, making this just an
> optimization of commit 87c00572ba05aa8c ("kvm: x86: emulate
> monitor and mwait instructions as nop").
>
> We can add it for 4.13 together with the idle driver.

I think that's a perfectly reasonable approach, yes. We can always add 
the PV flag with the driver.

Thanks a lot!


Alex