Re: [Qemu-devel] [PATCH v3 04/13] acpi: cphp: force switch to modern cpu hotplug if APIC ID > 254

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 18 Oct 2016 13:05:51 -0200
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Tue, Oct 18, 2016 at 04:34:53PM +0200, Igor Mammedov wrote:
> > On Tue, 18 Oct 2016 11:38:31 -0200
> > Eduardo Habkost <ehabkost@redhat.com> wrote:
> >   
> > > On Thu, Oct 13, 2016 at 11:52:38AM +0200, Igor Mammedov wrote:  
> > > > Switch to modern cpu hotplug at machine startup time if
> > > > a cpu present at boot has apic-id in range unsupported
> > > > by legacy cpu hotplug interface (i.e. > 254), to avoid
> > > > killing QEMU from legacy cpu hotplug code with error:
> > > >    "acpi: invalid cpu id: #apic-id#"
> > > > 
> > > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > > > ---
> > > >  hw/acpi/cpu_hotplug.c | 10 ++++++----
> > > >  1 file changed, 6 insertions(+), 4 deletions(-)
> > > > 
> > > > diff --git a/hw/acpi/cpu_hotplug.c b/hw/acpi/cpu_hotplug.c
> > > > index e19d902..c2ab9b8 100644
> > > > --- a/hw/acpi/cpu_hotplug.c
> > > > +++ b/hw/acpi/cpu_hotplug.c
> > > > @@ -63,7 +63,8 @@ static void acpi_set_cpu_present_bit(AcpiCpuHotplug *g, CPUState *cpu,
> > > >  
> > > >      cpu_id = k->get_arch_id(cpu);
> > > >      if ((cpu_id / 8) >= ACPI_GPE_PROC_LEN) {
> > > > -        error_setg(errp, "acpi: invalid cpu id: %" PRIi64, cpu_id);
> > > > +        object_property_set_bool(g->device, false, "cpu-hotplug-legacy",
> > > > +                                 &error_abort);    
> > > 
> > > What happens we are in legacy mode and this is triggered during
> > > hotplug instead of machine init? Would it break something, or is
> > > it safe?  
> >  case 1: guest with legacy hotplug AML (migrated for example) would use
> >          legacy interface and it won't be possible to trigger this path
> >          as target should be started with the same CLI as source
> >          (hence < 255 cpus)
> >  case 2: guest started on new QEMU will have new hotplug AML which switches
> >          QEMU to modern cpu hotplug interface at ACPI tables _INI time
> >          so this path is unreachable.  
> 
> I see. Thanks for the explanation!
> 
> > 
> > Originally it's been static rule:
> >     since 2.7 use new hotplug interface and old one for older machine types
> > Well it's complex but Michael insisted on keeping legacy hotplug
> > by default and do dynamic switching, so here we are.
> >   
> 
> This means PCMachineClass::legacy_cpu_hotplug means "legacy CPU
> hotplug _only_", because legacy CPU hotplug is always available
> on startup, right?
Sorry, I can't parse question, could you rephrase?

> 
> > this behavior is since 2.7:
> >     commit 679dd1a957df418453efdd3ed2914dba5cd73773
> >     pc: use new CPU hotplug interface since 2.7 machine type
> > 
> >    
> > > Unrelated to this patch: piix4_set_cpu_hotplug_legacy() has an
> > > assert(!value). I assume this means we must replace the QOM
> > > property with something that the user can't fiddle with, right?  
> > it's readonly to user after machine starts, but allows user
> > to play modern hotplug interface on old machine types if needed.
> > assert is there to trap mistake of switching to legacy mode
> > (which is default) from compat_properties.  
> 
> What exactly makes the property read-only to the user? Maybe we
> should make the setter return an error instead, as all
> object_property_set_bool("cpu-hotplug-legacy") calls already use
> &error_abort?
My mistake,
it's dynamic property with custom setters in piix4/ich9_pm and user
potentially can write there.

I was under impression that errors are ignored if property comes from
compat_props, if returning error would prevent machine from starting
when property comes from compat_props I can fix cpu-hotplug-legacy to
return error.

> 
> >   
> > >   
> > > >          return;
> > > >      }
> > > >  
> > > > @@ -85,13 +86,14 @@ void legacy_acpi_cpu_hotplug_init(MemoryRegion *parent, Object *owner,
> > > >  {
> > > >      CPUState *cpu;
> > > >  
> > > > -    CPU_FOREACH(cpu) {
> > > > -        acpi_set_cpu_present_bit(gpe_cpu, cpu, &error_abort);
> > > > -    }
> > > >      memory_region_init_io(&gpe_cpu->io, owner, &AcpiCpuHotplug_ops,
> > > >                            gpe_cpu, "acpi-cpu-hotplug", ACPI_GPE_PROC_LEN);
> > > >      memory_region_add_subregion(parent, base, &gpe_cpu->io);
> > > >      gpe_cpu->device = owner;
> > > > +
> > > > +    CPU_FOREACH(cpu) {
> > > > +        acpi_set_cpu_present_bit(gpe_cpu, cpu, &error_abort);
> > > > +    }
> > > >  }
> > > >  
> > > >  void acpi_switch_to_modern_cphp(AcpiCpuHotplug *gpe_cpu,
> > > > -- 
> > > > 2.7.4
> > > >     
> > >   
> >   
>