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Date: Wed, 13 Jun 2018 19:34:53 -0300
From: Marcelo Tosatti <mtosatti@redhat.com>
Message-ID: <20180613223449.GA18339@amt.cnet>
References: <20180612184616.90838-1-mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2 0/2] kvm: x86 CPU power management
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: qemu-devel@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>, Richard Henderson <rth@twiddle.net>, Eduardo Habkost <ehabkost@redhat.com>, kvm@vger.kernel.org

On Tue, Jun 12, 2018 at 09:47:11PM +0300, Michael S. Tsirkin wrote:
> This adds ability to expose host CPU power management capabilities to
> guests. For intel guests, this is sufficient for guest to enable
> low power CPU power management. For AMD guests it isn't sufficient,
> deeper C-states are entered using System-IO.
> 
> mwait based power management is tied closely to specifics of CPUID,
> making migration challenging. At this point only the non-migrateable
> -cpu host is supported.
> 
> With this patch applied, VM latency is within the noise of
> baremetal for some benchmarks.
> 
> perf bench sched pipe results:
> Before:
>     6.452 sec
> After:
>     4.382 sec
> Baremetal:
>     4.136 sec
> 
> Michael S. Tsirkin (2):
>   kvm: support -realtime cpu-pm=on|off
>   i386/cpu: make -cpu host support monitor/mwait
> 
>  include/sysemu/sysemu.h |  1 +
>  target/i386/cpu.h       |  9 +++++++++
>  target/i386/cpu.c       | 19 ++++++++++++++-----
>  target/i386/kvm.c       | 30 ++++++++++++++++++++++++++++++
>  vl.c                    |  6 ++++++
>  qemu-options.hx         |  9 +++++++--
>  6 files changed, 67 insertions(+), 7 deletions(-)
> 
> -- 
> MST

Hi Michael,

1) Command line option interface

Why is this not an optional cpu feature such as the other features? 


-cpu CPU,+mwait  

rather than a separate, architecture independent "-realtime cpu-pm=on|off" 
command line option?

2) Migration

Isnt it sufficient to check that both CPUID leafs are the same, 
to allow migration ?

1. Check that the processor supports MONITOR and MWAIT. If
CPUID.01H:ECX.MONITOR[bit 3] = 1, MONITOR and MWAIT are available at
ring 0.

2. Query the smallest and largest line size that MONITOR uses.
Use CPUID.05H:EAX.smallest[bits 15:0];EBX.largest[bits15:0].