Re: [PATCH 3/5] hw/ppc/e500: QOM-attach CPUs to the machine container

[Markus Armbruster <armbru@redhat.com>]

Philippe Mathieu-Daudé <philmd@linaro.org> writes:

> On 3/11/23 08:40, Markus Armbruster wrote:
>> Philippe Mathieu-Daudé <philmd@linaro.org> writes:
>> 
>>> Instead of having CPUs dangling in the /unattached/device
>>> bucket, attach them to the machine container.
>>>
>>> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>>> ---
>>>   hw/ppc/e500.c | 1 +
>>>   1 file changed, 1 insertion(+)
>>>
>>> diff --git a/hw/ppc/e500.c b/hw/ppc/e500.c
>>> index e04114fb3c..f8177c0280 100644
>>> --- a/hw/ppc/e500.c
>>> +++ b/hw/ppc/e500.c
>>> @@ -946,6 +946,7 @@ void ppce500_init(MachineState *machine)
>>>               exit(1);
>>>           }
>>>   +        object_property_add_child(OBJECT(machine), "cpu[*]", OBJECT(cs));
>>>           /*
>>>            * Secondary CPU starts in halted state for now. Needs to change
>>>            * when implementing non-kernel boot.
>> A peek at "info qom-tree" confirms the CPU is in /machine/unattached/.
>> Along with most onboard devices.  Details below.
>>
>> Quick test...  I count 563 machines.  394 seem to have CPU(s) in or
>> below /machine/unattached/, 129 elsewhere, and 40 I can't easily
>> examine, because they don't start to monitor without additional CLI
>> arguments.
>>
>> Where should CPUs be?
>
> It is machine specific.
>
> - For System-on-Chip, it would be in /soc
>
> - For systems that fully model CPU topology, I'd expect a consistent
>   topology path. (If it is part of a cluster, in that /cluster).
>
> - For mainframes, it should be part of the CPU cards that can be
>   inserted?
>
> - For a single Pentium CPU, maybe /machine is sufficient.
>
>> Is /machine/unattached/ basically where we dump products of lazy
>> modelling?
>
> Unfortunately, yes. Also where CLI created devices are I guess.

No, these go into /machine/peripheral/ (with id=...) or
/machine/peripheral-anon/ (without).

/unattached has a different role: it's where objects without a parent go
when a parent is needed.  For instance, when a device without a QOM
parent gets realized, device_set_realized() makes it a child of
/unattached/.  Similar logic in hw/core/gpio.c, system/ioport.c and
system/memory.c.

>> If yes, should we try to empty it out?
>
> If it is useful. For components expected to be referenced externally
> by humans, probably. If only used by scripts, maybe not, except if
> human have to debug.
>
>> If we shouldn't, then why move this one out?
>
> When looking for a component in the tree, I start to look at /machine,
> having to fish for it elsewhere is not very natural. I'd change your
> question by:
> - Why do we need /unattached?

Because we can't be bothered to pick parents?

Perhaps an excusable shortcut when we had to convert a big pile of
devices to QOM.  But we take the shortcut for new objects, too.
Surprise, surprise.

> or
> - Why do we have 2 different folders, /machine and /unattached?
> If it is a headache, why not just simply merge them both?

I guess a justification for having both could be:

    /machine/: somebody spent a brain wave or two on the proper parent

    /unattached/: what's a parent, and why should I care?

Merging them would lose information.  Do we care?

>> $ qemu-system-ppc -nodefaults -S -display none -M ppce500 -monitor stdio
>> QEMU 8.1.50 monitor - type 'help' for more information
>> (qemu) info qom-tree
>> /machine (ppce500-machine)
>>    /e500-ccsr (e500-ccsr)
>>      /e500-ccsr[0] (memory-region)
>>      /e500-pci-bar0[0] (memory-region)
>>    /pci-host (e500-pcihost)
>>      /bm-e500[0] (memory-region)
>>      /pci bus memory[0] (memory-region)
>>      /pci-conf-data[0] (memory-region)
>>      /pci-conf-idx[0] (memory-region)
>>      /pci-container[0] (memory-region)
>>      /pci-pio[0] (memory-region)
>>      /pci.0 (PCI)
>>      /pci.reg[0] (memory-region)
>>    /peripheral (container)
>>    /peripheral-anon (container)
> [...]
>
>>    /unattached (container)
>>      /device[0] (e500v2_v30-powerpc-cpu)
>>        /unnamed-gpio-in[0] (irq)
>>        /unnamed-gpio-in[1] (irq)
>>        /unnamed-gpio-in[2] (irq)
>>        /unnamed-gpio-in[3] (irq)
>>        /unnamed-gpio-in[4] (irq)
>>        /unnamed-gpio-in[5] (irq)
>>        /unnamed-gpio-in[6] (irq)
>>      /device[1] (mpc-i2c)
>>        /i2c (i2c-bus)
>>        /mpc-i2c[0] (memory-region)
>>      /device[2] (ds1338)
>>      /device[3] (unimplemented-device)
>>        /esdhc[0] (memory-region)
>>      /device[4] (generic-sdhci)
>>        /sd-bus (sdhci-bus)
>>        /sdhci[0] (memory-region)
>>      /device[5] (mpc8544-guts)
>>        /mpc8544.guts[0] (memory-region)
>>      /device[6] (e500-host-bridge)
>>        /bus master container[0] (memory-region)
>>        /bus master[0] (memory-region)
>>      /device[7] (e500-spin)
>>        /e500 spin pv device[0] (memory-region)
>>      /device[8] (mpc8xxx_gpio)
>>        /mpc8xxx_gpio[0] (memory-region)
>>        /unnamed-gpio-in[0] (irq)
>>        /unnamed-gpio-in[10] (irq)
>>        /unnamed-gpio-in[11] (irq)
>>        /unnamed-gpio-in[12] (irq)
>>        /unnamed-gpio-in[13] (irq)
>>        /unnamed-gpio-in[14] (irq)
>>        /unnamed-gpio-in[15] (irq)
>>        /unnamed-gpio-in[16] (irq)
>>        /unnamed-gpio-in[17] (irq)
>>        /unnamed-gpio-in[18] (irq)
>>        /unnamed-gpio-in[19] (irq)
>>        /unnamed-gpio-in[1] (irq)
>>        /unnamed-gpio-in[20] (irq)
>>        /unnamed-gpio-in[21] (irq)
>>        /unnamed-gpio-in[22] (irq)
>>        /unnamed-gpio-in[23] (irq)
>>        /unnamed-gpio-in[24] (irq)
>>        /unnamed-gpio-in[25] (irq)
>>        /unnamed-gpio-in[26] (irq)
>>        /unnamed-gpio-in[27] (irq)
>>        /unnamed-gpio-in[28] (irq)
>>        /unnamed-gpio-in[29] (irq)
>>        /unnamed-gpio-in[2] (irq)
>>        /unnamed-gpio-in[30] (irq)
>>        /unnamed-gpio-in[31] (irq)
>>        /unnamed-gpio-in[3] (irq)
>>        /unnamed-gpio-in[4] (irq)
>>        /unnamed-gpio-in[5] (irq)
>>        /unnamed-gpio-in[6] (irq)
>>        /unnamed-gpio-in[7] (irq)
>>        /unnamed-gpio-in[8] (irq)
>>        /unnamed-gpio-in[9] (irq)
>>      /device[9] (platform-bus-device)
>>        /platform bus[0] (memory-region)
>
> Actually most of these do have a QOM parent.

They don't or else they wouldn't be here.  Do you mean "the proper
parent is obvious"?

> Correctly placing them in the tree should help when trying to
> resolve a component and avoiding an ambiguous match.

Yes.

>>      /io[0] (memory-region)
>>      /non-qdev-gpio[0] (irq)
>>      /sysbus (System)
>>      /system[0] (memory-region)