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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v6] kvm: better MWAIT emulation for guests
Date: Fri, 21 Apr 2017 12:06:49 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

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.


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