Re: [Qemu-devel] [RFC v2 3/6] possible_cpus: add CPUArchId::type field

[David Hildenbrand <david@redhat.com>]

On 10.11.2017 11:14, Cornelia Huck wrote:
> On Thu, 9 Nov 2017 18:02:35 -0200
> Eduardo Habkost <ehabkost@redhat.com> wrote:
> 
>> On Thu, Nov 09, 2017 at 05:58:03PM +1100, David Gibson wrote:
>>> On Tue, Nov 07, 2017 at 04:04:04PM +0100, Cornelia Huck wrote:  
>>>> On Mon, 6 Nov 2017 16:02:16 -0200
>>>> Eduardo Habkost <ehabkost@redhat.com> wrote:
>>>>   
>>>>> On Tue, Oct 31, 2017 at 03:01:14PM +0100, Igor Mammedov wrote:  
>>>>>> On Thu, 19 Oct 2017 17:31:51 +1100
>>>>>> David Gibson <david@gibson.dropbear.id.au> wrote:
>>>>>>     
>>>>>>> On Wed, Oct 18, 2017 at 01:12:12PM +0200, Igor Mammedov wrote:    
>>>>>>>> For enabling early cpu to numa node configuration at runtime
>>>>>>>> qmp_query_hotpluggable_cpus() should provide a list of available
>>>>>>>> cpu slots at early stage, before machine_init() is called and
>>>>>>>> the 1st cpu is created, so that mgmt might be able to call it
>>>>>>>> and use output to set numa mapping.
>>>>>>>> Use MachineClass::possible_cpu_arch_ids() callback to set
>>>>>>>> cpu type info, along with the rest of possible cpu properties,
>>>>>>>> to let machine define which cpu type* will be used.
>>>>>>>>
>>>>>>>> * for SPAPR it will be a spapr core type and for ARM/s390x/x86
>>>>>>>>   a respective descendant of CPUClass.
>>>>>>>>
>>>>>>>> Move parse_numa_opts() in vl.c after cpu_model is parsed into
>>>>>>>> cpu_type so that possible_cpu_arch_ids() would know which
>>>>>>>> cpu_type to use during layout initialization.
>>>>>>>>
>>>>>>>> Signed-off-by: Igor Mammedov <imammedo@redhat.com>      
>>>>>>>
>>>>>>> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
>>>>>>>     
>>>>>>>> ---
>>>>>>>>   v2:
>>>>>>>>      - fix NULL dereference caused by not initialized
>>>>>>>>        MachineState::cpu_type at the time parse_numa_opts()
>>>>>>>>        were called
>>>>>>>> ---
>>>>>>>>  include/hw/boards.h        |  2 ++
>>>>>>>>  hw/arm/virt.c              |  3 ++-
>>>>>>>>  hw/core/machine.c          | 12 ++++++------
>>>>>>>>  hw/i386/pc.c               |  4 +++-
>>>>>>>>  hw/ppc/spapr.c             | 13 ++++++++-----
>>>>>>>>  hw/s390x/s390-virtio-ccw.c |  1 +
>>>>>>>>  vl.c                       |  3 +--
>>>>>>>>  7 files changed, 23 insertions(+), 15 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/include/hw/boards.h b/include/hw/boards.h
>>>>>>>> index 191a5b3..fa21758 100644
>>>>>>>> --- a/include/hw/boards.h
>>>>>>>> +++ b/include/hw/boards.h
>>>>>>>> @@ -80,6 +80,7 @@ void machine_set_cpu_numa_node(MachineState *machine,
>>>>>>>>   * CPUArchId:
>>>>>>>>   * @arch_id - architecture-dependent CPU ID of present or possible CPU      
>>>>>>>
>>>>>>> I know this isn't really in scope for this patch, but is @arch_id here
>>>>>>> supposed to have meaning defined by the target, or by the machine?
>>>>>>>
>>>>>>> If it's the machime, it could do with a rename - "arch" means target
>>>>>>> to most people (thanks to Linux).
>>>>>>>
>>>>>>> If it's the target, it's kind of bogus, because it doesn't necessarily
>>>>>>> have a clear meaning per target - get_arch_id in CPUClass has the same
>>>>>>> problem, which is probably one reason it's basically only used by the
>>>>>>> x86 code at present.
>>>>>>>
>>>>>>> e.g. for target/ppc, what do we use?  There's the PIR, which is in the
>>>>>>> CPU.. but only on some cpu models, not all.  There will generally be
>>>>>>> some kind of master PIC id, but there are different PIC models on
>>>>>>> different boards.  What goes in the devicetree?  Well only some
>>>>>>> machines use devicetree, and they might define the cpu reg 
>>>>>>> differently.
>>>>>>>
>>>>>>> Board designs will generally try to make some if not all of those
>>>>>>> possible values equal for simplicity, but there's still no real way of
>>>>>>> defining a sensible arch_id independent of machine / board.    
>>>>>> I'd say arch_id is machine specific so far, it was introduced when we
>>>>>> didn't have CpuInstanceProperties and at that time we considered only
>>>>>> vcpus (threads) and doesn't really apply to spapr cores.
>>>>>>
>>>>>> In general we could do away with arch_id and use CpuInstanceProperties
>>>>>> instead, but arch_id also serves aux purpose, it allows machine to
>>>>>> pre-calculate(cache) apic-id/mpidr values in one place and then they
>>>>>> are/(could be) used by arch in-depended code to build acpi tables.
>>>>>> So if we drop arch_id we would need to introduce a machine hook,
>>>>>> which would translate CpuInstanceProperties into current arch_id.    
>>>>>
>>>>> I think we need to do a better to job documenting where exactly
>>>>> we expect arch_id to be used and how, so people know what it's
>>>>> supposed to return.
>>>>>
>>>>> If the only place where it's useful now is ACPI code (is it?),
>>>>> should we rename it to something like get_acpi_id()?  
>>>>
>>>> It is also used in hw/s390x/sclp.c to fill out a control block, so acpi
>>>> isn't the only user.  
>>>
>>> Yeah.. this is kind of bogus.  The s390 use is in machine specific
>>> code, so it's basically just re-using the field for an unrelated usage
>>> to the x86/arm one (ACPI).

as index == arch_id on s390x, that code could easily be changed to
something like:

@@ -45,7 +45,7 @@ static void prepare_cpu_entries(SCLPDevice *sclp,
CPUEntry *entry, int *count)
         if (!ms->possible_cpus->cpus[i].cpu) {
             continue;
         }
-        entry[*count].address = ms->possible_cpus->cpus[i].arch_id;
+        entry[*count].address = i;
         entry[*count].type = 0;
         memcpy(entry[*count].features, features, sizeof(features));
         (*count)++;

arch_id just looked like the right thing to use (documentation issue
mentioned above)


>>>
>>> If we can't assign a universal meaning to the field (even if the
>>> actual values are per-machine) - and I don't think we can - then I
>>> really don't think it belongs in CPUState.  A machine hook which
>>> translates an ArchId to an acpi_id is the correct solution I believe.
>>> Or even an ACPIMachine interface (to be implemented by machines which
>>> do ACPI) which has a method to do this.
>>>
>>> Since both the assignment and use are in machine type specific code
>>> for s390, it can have its own field in the s390 specific cpu subclass.

s390x doesn't need arch_id at all.

cs->cpu_index can be used.

>>>   
>>
>> I agree.  This might require duplicating cpu_by_arch_id() and
>> cpu_exists() into machine-specific code, but this doesn't sound
>> too bad: there's only one user of cpu_by_arch_id() (that's
>> x86-specific code living inside monitor.c), and one user of
>> cpu_exists() (that's s390-specific code).>>
>> (Maybe those users could be rewritten to use
>> MachineState::possible_cpus, like pc_find_cpu_slot()).


-- 

Thanks,

David / dhildenb