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

[Eduardo Habkost]

On Tue, Oct 18, 2016 at 04:34:53PM +0200, Igor Mammedov wrote:
> On Tue, 18 Oct 2016 11:38:31 -0200
> Eduardo Habkost <address@hidden> 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 <address@hidden>
> > > ---
> > >  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?

> 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?

> 
> > 
> > >          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
> > >   
> > 
> 

-- 
Eduardo