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

Eduardo Habkost
2017-01-04T13:40:23Z

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.

>
> 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