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

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Date:   Fri, 21 Apr 2017 12:05:10 +0200
From:   Alexander Graf <agraf@...e.de>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "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 21.04.17 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