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]. >
