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

[Marcelo Tosatti]

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?

> > 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 <address@hidden>
> > > ---
> > >  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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