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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 12:02:53 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

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.


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