Re: [Qemu-devel] [RFC PATCH v2 2/5] cpu: Introduce CPUState::stable_cpu_id

[Igor Mammedov]

On Mon, 11 Jul 2016 13:22:37 +1000
David Gibson <address@hidden> wrote:

> On Fri, Jul 08, 2016 at 01:11:02PM +0200, Igor Mammedov wrote:
> > On Fri, 8 Jul 2016 15:19:58 +1000
> > David Gibson <address@hidden> wrote:
> >   
> > > On Thu, Jul 07, 2016 at 08:20:22PM +0530, Bharata B Rao wrote:  
> > > > Add CPUState::stable_cpu_id and use that as instance_id in
> > > > vmstate_register() call.
> > > > 
> > > > Introduce has-stable_cpu_id property that allows target machines to
> > > > optionally switch to using stable_cpu_id instead of cpu_index.
> > > > This will help allow successful migration in cases where holes are
> > > > introduced in cpu_index range after CPU hot removals.
> > > > 
> > > > Suggested-by: Igor Mammedov <address@hidden>
> > > > Signed-off-by: Bharata B Rao <address@hidden>
> > > > ---
> > > >  exec.c            | 6 ++++--
> > > >  include/qom/cpu.h | 5 +++++
> > > >  qom/cpu.c         | 6 ++++++
> > > >  3 files changed, 15 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/exec.c b/exec.c
> > > > index fb73910..3b36fe5 100644
> > > > --- a/exec.c
> > > > +++ b/exec.c
> > > > @@ -619,12 +619,14 @@ static void cpu_release_index(CPUState *cpu)
> > > >  void cpu_vmstate_register(CPUState *cpu)
> > > >  {
> > > >      CPUClass *cc = CPU_GET_CLASS(cpu);
> > > > +    int instance_id = cpu->has_stable_cpu_id ? cpu->stable_cpu_id :
> > > > +                      cpu->cpu_index;
> > > >  
> > > >      if (qdev_get_vmsd(DEVICE(cpu)) == NULL) {
> > > > -        vmstate_register(NULL, cpu->cpu_index, &vmstate_cpu_common, 
> > > > cpu);
> > > > +        vmstate_register(NULL, instance_id, &vmstate_cpu_common, cpu);
> > > >      }
> > > >      if (cc->vmsd != NULL) {
> > > > -        vmstate_register(NULL, cpu->cpu_index, cc->vmsd, cpu);
> > > > +        vmstate_register(NULL, instance_id, cc->vmsd, cpu);
> > > >      }
> > > >  }
> > > >  
> > > > diff --git a/include/qom/cpu.h b/include/qom/cpu.h
> > > > index 331386f..527c021 100644
> > > > --- a/include/qom/cpu.h
> > > > +++ b/include/qom/cpu.h
> > > > @@ -273,6 +273,9 @@ struct qemu_work_item {
> > > >   * @kvm_fd: vCPU file descriptor for KVM.
> > > >   * @work_mutex: Lock to prevent multiple access to queued_work_*.
> > > >   * @queued_work_first: First asynchronous work pending.
> > > > + * @stable_cpu_id: Use as instance_id argument in cpu vmstate_register 
> > > > calls
> > > > + * @has_stable_cpu_id: Set to enforce the use of @stable_cpu_id
> > > > + *     over cpu_index during vmstate registration.
> > > >   *
> > > >   * State of one CPU core or thread.
> > > >   */
> > > > @@ -360,6 +363,8 @@ struct CPUState {
> > > >         (absolute value) offset as small as possible.  This reduces code
> > > >         size, especially for hosts without large memory offsets.  */
> > > >      uint32_t tcg_exit_req;
> > > > +    int stable_cpu_id;
> > > > +    bool has_stable_cpu_id;
> > > >  };
> > > >  
> > > >  QTAILQ_HEAD(CPUTailQ, CPUState);
> > > > diff --git a/qom/cpu.c b/qom/cpu.c
> > > > index 1095ea1..bae1bf7 100644
> > > > --- a/qom/cpu.c
> > > > +++ b/qom/cpu.c
> > > > @@ -363,6 +363,11 @@ static int64_t cpu_common_get_arch_id(CPUState 
> > > > *cpu)
> > > >      return cpu->cpu_index;
> > > >  }
> > > >  
> > > > +static Property cpu_common_properties[] = {
> > > > +    DEFINE_PROP_BOOL("has-stable-cpu-id", CPUState, has_stable_cpu_id, 
> > > > false),    
> > > 
> > > It seems odd to me that stable_cpu_id itself isn't exposed as a
> > > property.  Even if we don't need to set it externally for now, it
> > > really should be QOM introspectable.  
> > Should it? Why?  
> 
> Well, for one thing it's really strange to have the boolean flag
> exposed, but not the value itself.
property doesn't always means that it's intended as an external interface

> 
> > It's QEMU internal detail and outside world preferably shouldn't
> > know anything about it.  
> 
> Hrm.. I guess kinda.  But I think it's less an internal detail than
> the existing cpu_index is.
so it' better not to start to advertise it as an external interface.

Should be add some flag to generic property to mark it as internal?

> 
> > As example look at cpu_index which were/is used in HMP and -numa
> > interfaces and now mgmt tries make some sense of it.
> > 
> > Cleaner way should be teaching -numa to handle cpus specified by
> > socket/core/thread IDs so that mgmt would actually know what CPUs
> > it assigns to what nodes and not to look at/invent/generate some
> > internal cpu_id.
> >   
> > >   
> > > > +    DEFINE_PROP_END_OF_LIST()
> > > > +};
> > > > +
> > > >  static void cpu_class_init(ObjectClass *klass, void *data)
> > > >  {
> > > >      DeviceClass *dc = DEVICE_CLASS(klass);
> > > > @@ -394,6 +399,7 @@ static void cpu_class_init(ObjectClass *klass, void 
> > > > *data)
> > > >       * IRQs, adding reset handlers, halting non-first CPUs, ...
> > > >       */
> > > >      dc->cannot_instantiate_with_device_add_yet = true;
> > > > +    dc->props = cpu_common_properties;
> > > >  }
> > > >  
> > > >  static const TypeInfo cpu_type_info = {    
> > >   
> >   
>