Re: [PATCH 2/4] kvm: Allow invtsc migration if tsc-khz is set explicitly

Eduardo Habkost
2017-01-04T13:51:39Z

On Wed, Jan 04, 2017 at 11:45:36AM -0200, Marcelo Tosatti wrote:
> On Wed, Jan 04, 2017 at 11:40:17AM -0200, Eduardo Habkost wrote:
> > On Wed, Jan 04, 2017 at 09:44:06AM -0200, Marcelo Tosatti wrote:
> > > On Tue, Dec 27, 2016 at 05:21:18PM -0200, Eduardo Habkost wrote:
> > > > We can safely allow a VM to be migrated with invtsc enabled if
> > > > tsc-khz is set explicitly, because QEMU already refuses to start
> > > > if it can't set the TSC frequency to the configured value.
> > >
> > > On the source host, yes, but on the destination it does not.
> > >
> > > SOURCE: -cpu kvm64,tsc-frequency=2660001000
> > > DESTINATION: -cpu kvm64
> >
> > This is invalid usage. Management software is responsible for
> > keeping the same configuration on source and destination.
>
> Fine. Just to confirm: you do not expect libvirt to
> use
>
> -cpu XXX,tsc-frequency=ZZZZZ
>
> On both source and destination, correct?

They can omit it, as long as it is omitted on both source and
destination.

If they do use tsc-frequency on the source, they must use it on
the destination as well. Just like they do for every device
option in QEMU.

>
> > > Only prints:
> > >
> > > qemu-system-x86_64: warning: TSC frequency mismatch between VM (2660001
> > > kHz) and host (2893428 kHz), and TSC scaling unavailable
> > > qemu-system-x86_64: warning: TSC frequency mismatch between VM (2660001
> > > kHz) and host (2893428 kHz), and TSC scaling unavailable
> > >
> > > And allows migration.
> > >
> > > > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> > > > ---
> > > > target/i386/kvm.c | 19 ++++++++++---------
> > > > 1 file changed, 10 insertions(+), 9 deletions(-)
> > > >
> > > > diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> > > > index a26290f..6a51399 100644
> > > > --- a/target/i386/kvm.c
> > > > +++ b/target/i386/kvm.c
> > > > @@ -962,15 +962,16 @@ int kvm_arch_init_vcpu(CPUState *cs)
> > > > has_msr_mcg_ext_ctl = has_msr_feature_control = true;
> > > > }
> > > >
> > > > - if ((env->features[FEAT_8000_0007_EDX] & CPUID_APM_INVTSC) &&
> > > > - invtsc_mig_blocker == NULL) {
> > > > - /* for migration */
> > > > - error_setg(&invtsc_mig_blocker,
> > > > - "State blocked by non-migratable CPU device"
> > > > - " (invtsc flag)");
> > > > - migrate_add_blocker(invtsc_mig_blocker);
> > > > - /* for savevm */
> > > > - vmstate_x86_cpu.unmigratable = 1;
> > > > + if (!env->user_tsc_khz) {
> > > > + if ((env->features[FEAT_8000_0007_EDX] & CPUID_APM_INVTSC) &&
> > > > + invtsc_mig_blocker == NULL) {
> > > > + /* for migration */
> > > > + error_setg(&invtsc_mig_blocker,
> > > > + "State blocked by non-migratable CPU device"
> > > > + " (invtsc flag)");
> > > > + migrate_add_blocker(invtsc_mig_blocker);
> > > > + /* for savevm */
> > > > + vmstate_x86_cpu.unmigratable = 1;
> > > > }
> > > >
> > > > cpuid_data.cpuid.padding = 0;
> > > > --
> > > > 2.7.4
> >
> > --
> > Eduardo
> > --
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--
Eduardo