Re: [Qemu-devel] [PATCH v2 0/2] kvm: x86 CPU power management

["Michael S. Tsirkin" <mst@redhat.com>]

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.

> 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.

> 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]. 
>