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

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Date:   Fri, 21 Apr 2017 12:06:49 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Alexander Graf <agraf@...e.de>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jim Mattson <jmattson@...gle.com>, kvm list <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Gabriel L. Somlo" <gsomlo@...il.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>, linux-doc@...r.kernel.org,
        qemu-devel@...gnu.org
Subject: Re: [PATCH v6] kvm: better MWAIT emulation for guests



On 21/04/2017 12:05, Alexander Graf wrote:
> 
> 
> 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!

Queuing the patch for 4.12.

Paolo