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Re: [libvirt] Exposing and calculating CPU APIC IDs (was Re: [Qemu-devel] [RFC 1/3] target-i386: moving registers of vmstate from cpu_exec_init() to x86_cpu_realizefn())



On Mon, 2014-01-20 at 13:29 +0100, Igor Mammedov wrote:
> On Fri, 17 Jan 2014 17:13:55 -0200
> Eduardo Habkost <ehabkost redhat com> wrote:
> 
> > On Wed, Jan 15, 2014 at 03:37:04PM +0100, Igor Mammedov wrote:
> > > On Wed, 15 Jan 2014 20:24:01 +0800
> > > Chen Fan <chen fan fnst cn fujitsu com> wrote:
> > > > On Tue, 2014-01-14 at 11:40 +0100, Igor Mammedov wrote:
> > > > > On Tue, 14 Jan 2014 17:27:20 +0800
> > > > > Chen Fan <chen fan fnst cn fujitsu com> wrote:
> > > > > 
> > > > > > the intend of this patch is to register cpu vmstates with apic id instead of cpu
> > > > > > index, due to the property setting of apic_id is behind the cpu initialization. so
> > > > > > we move the registers of cpu vmstate from cpu_exec_init() to x86_cpu_realizefn() to
> > > > > > let the set apicid as the parameter.
> > > > > > 
> > > > > > Signed-off-by: Chen Fan <chen fan fnst cn fujitsu com>
> > > > > > ---
> > > > > >  exec.c            | 5 +++++
> > > > > >  target-i386/cpu.c | 9 +++++++++
> > > > > >  2 files changed, 14 insertions(+)
> > > > > > 
> > > > > > diff --git a/exec.c b/exec.c
> > > > > > index 7e49e8e..9be5855 100644
> > > > > > --- a/exec.c
> > > > > > +++ b/exec.c
> > > > > > @@ -438,7 +438,9 @@ CPUState *qemu_get_cpu(int index)
> > > > > >  void cpu_exec_init(CPUArchState *env)
> > > > > >  {
> > > > > >      CPUState *cpu = ENV_GET_CPU(env);
> > > > > > +#if !defined(TARGET_I386)
> > > > > >      CPUClass *cc = CPU_GET_CLASS(cpu);
> > > > > > +#endif
> > > > > >      CPUState *some_cpu;
> > > > > >      int cpu_index;
> > > > > >  
> > > > > > @@ -460,6 +462,8 @@ void cpu_exec_init(CPUArchState *env)
> > > > > >  #if defined(CONFIG_USER_ONLY)
> > > > > >      cpu_list_unlock();
> > > > > >  #endif
> > > > > > +
> > > > > > +#if !defined(TARGET_I386)
> > > > > >      if (qdev_get_vmsd(DEVICE(cpu)) == NULL) {
> > > > > >          vmstate_register(NULL, cpu_index, &vmstate_cpu_common, cpu);
> > > > > >      }
> > > > > > @@ -472,6 +476,7 @@ void cpu_exec_init(CPUArchState *env)
> > > > > >      if (cc->vmsd != NULL) {
> > > > > >          vmstate_register(NULL, cpu_index, cc->vmsd, cpu);
> > > > > >      }
> > > > > > +#endif /* !TARGET_I386 */
> > > > > >  }
> > > > > >  
> > > > > >  #if defined(TARGET_HAS_ICE)
> > > > > > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > > > > > index 967529a..dada6f6 100644
> > > > > > --- a/target-i386/cpu.c
> > > > > > +++ b/target-i386/cpu.c
> > > > > > @@ -2552,6 +2552,7 @@ static void x86_cpu_apic_realize(X86CPU *cpu, Error **errp)
> > > > > >  static void x86_cpu_realizefn(DeviceState *dev, Error **errp)
> > > > > >  {
> > > > > >      CPUState *cs = CPU(dev);
> > > > > > +    CPUClass *cc = CPU_GET_CLASS(cs);
> > > > > >      X86CPU *cpu = X86_CPU(dev);
> > > > > >      X86CPUClass *xcc = X86_CPU_GET_CLASS(dev);
> > > > > >      CPUX86State *env = &cpu->env;
> > > > > > @@ -2615,6 +2616,14 @@ static void x86_cpu_realizefn(DeviceState *dev, Error **errp)
> > > > > >      cpu_reset(cs);
> > > > > >  
> > > > > >      xcc->parent_realize(dev, &local_err);
> > > > > > +
> > > > > > +    if (qdev_get_vmsd(DEVICE(cs)) == NULL) {
> > > > > > +        vmstate_register(NULL, env->cpuid_apic_id, &vmstate_cpu_common, cs);
> > > > > > +    }
> > > > > > +
> > > > > > +    if (cc->vmsd != NULL) {
> > > > > > +        vmstate_register(NULL, env->cpuid_apic_id, cc->vmsd, cs);
> > > > > > +    }
> > > > > how about doing it in common CPUclass.realize()
> > > > > you can use get_arch_id() for getting CPU id, it returns cpu_index by default
> > > > > and apic_id for target-i386.
> > > > 
> > > > Thanks for your kind suggestion, does this mean we can directly move
> > > > vmstate_register to cpu_common_realizefn()? 
> > > yep, that is a gist of it.
> > > 
> > > There is more to the issue with discontinuous CPUs, a lot of code still
> > > use cpu_index and the way it's allocated is not compatible with discontinuous
> > > CPUs, so this issue should be fixed as well.
> > > 
> > > Also you propose to use a raw apic id with CLI/user interface.
> > > I recall there were objections to it since APIC ID contains topology
> > > information and it's not trivial for user to get it right.
> > > The last idea that was discussed to fix it was not expose APIC ID to
> > > user but rather introduce QOM hierarchy like:
> > >   /machine/node/N/socket/X/core/Y/thread/Z
> > > and use it in user interface as a means to specify an arbitrary CPU
> > > and let QEMU calculate APIC ID based on this path.
> > > 
> > > But nobody took on implementing it yet.
> > 
> > We're taking so long to get a decent interface implemented, that part of
> > me is considering exposing the APIC ID directly like suggested before,
> > and requiring libvirt to calculate topology-aware APIC IDs[1] to
> > properly implement CPU hotplug (and possibly for other tasks).
> If you are speaking about 
> 'qemu will core dump with "-smp 254, sockets=2, cores=3, threads=2"'
> http://patchwork.ozlabs.org/patch/301272/
> bug then it's limitation of ACPI implementation,
> I'm going to refactor it to use full APIC ids instead of using bitmap,
> so that we won't ever run into issue regardless of cpu supported CPU count.
> 
> > 
> > Another part of me is hoping that the libvirt developers ask us to
> > please not do that, so I can use it as argument against exposing the
> > APIC IDs directly the next time we discuss this.  :)
> 
> why not try your  /machine/node/N/socket/X/core/Y/thread/Z idea first.
> It will benefit not only cpu hotplug but also '-numa' and topology
> description in general.
> 
have there been any plan/model of the idea? Need to add a new option to
qemu command?

Thanks,
Chen

> 
> > [1] See target-i386/topology.h for references
> > 
> 



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