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Re: [Qemu-devel] [PATCH v2 0/2] kvm: x86 CPU power management
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH v2 0/2] kvm: x86 CPU power management |
Date: |
Sat, 16 Jun 2018 00:53:40 +0300 |
On Fri, Jun 15, 2018 at 03:32:27PM -0300, Marcelo Tosatti wrote:
> On Thu, Jun 14, 2018 at 02:37:28AM +0300, Michael S. Tsirkin wrote:
> > On Wed, Jun 13, 2018 at 07:34:53PM -0300, Marcelo Tosatti wrote:
> > > 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?
> >
> > Because it's not just a guest flag. With guest pm on, one guest
> > can severely affect the latency of others on the same host CPU.
>
> How so ?
Look at drivers/idle/intel_idle.c
There are states with exit latencies of
10000
> > > 2) Migration
> > >
> > > Isnt it sufficient to check that both CPUID leafs are the same,
> > > to allow migration ?
> >
> > Not at the moment since linux guests use mwait hints and latency values
> > from a table in intel_idle. If the host and guest models do not match,
> > mwait will get a wrong hint.
> >
> > It will not do the right thing then!
> >
> > You want exactly the same host CPU for it to work.
> >
> > This isn't different from how -host cpu works generally.
>
> Ok, makes sense.
>
> > > 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].
> > >
Re: [Qemu-devel] [PATCH v2 0/2] kvm: x86 CPU power management, Kashyap Chamarthy, 2018/06/22