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

[Eduardo Habkost <ehabkost@redhat.com>]

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