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

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Date:   Tue, 4 Apr 2017 15:13:09 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Alexander Graf <agraf@...e.de>
Cc:     Jim Mattson <jmattson@...gle.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Gabriel L. Somlo" <gsomlo@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests

2017-04-04 14:51+0200, Alexander Graf:
> On 04/04/2017 02:39 PM, Radim Krčmář wrote:
>> 2017-04-03 12:04+0200, Alexander Graf:
>> > 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.

We have tried to pass host's CPUID MWAIT leaf and it still failed:
https://www.spinics.net/lists/kvm/msg146686.html

I wouldn't mind breaking that particular combination of OS X and
hardware, but I'm worried to do it because we don't understand why it
broke, so there could be more ...

>>   * 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.

Ah, makes sense.

>> 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 ;).

Nice.  MWAIT could also use setting of arbitrary values for its leaf,
but a generic interface for that would probably look clunky on the
command line ...