Re: [Qemu-devel] [PATCH] cpu: Register QOM links at /machine/cpus/<index>

[Paolo Bonzini]

On 04/05/2015 16:05, Eduardo Habkost wrote:
> On Mon, May 04, 2015 at 03:19:32PM +0200, Paolo Bonzini wrote:
>>
>>
>> On 04/05/2015 15:16, Igor Mammedov wrote:
>>>>> Can we use the APIC id then?  Perhaps wrapped with a CPUState-level
>>>>> method get_stable_processor_id()?
>>> We have CPUClass->get_arch_id() which results in APIC id for
>>> target-i386.
>>> But I'd rather see an arbitrary DEVICE->id as index/name, that way
>>> when -device cpu-foo,id=cpuXXX becomes functional we would have
>>> 1:1 mapping between CLI and /machine/cpus/ view.
>>
>> CPUs would already be available at /machine/peripheral.  I think aliases
>> should provide alternative indexing whenever possible---not simply
>> filter by device type.
> 
> [1] Is there anybody or any document that can explain to me what all the
>     containers inside /machine mean? I see /machine/peripheral,
>     /machine/peripheral-anon, /machine/unattached, here, and I don't
>     know what they mean.

/machine/peripheral/XYZ holds devices created with -device id=XYZ

/machine/peripheral-anon/device[NN] holds devices created without an id

/machine/unattached holds devices created by the board and not added
elsewhere through object_property_add_child.

> Could you clarify what you mean by "alternative indexing"?

A way to lookup devices of a particular kind.  An example of
"alternative indexing" is using pci.0/child[NN] to look up children of
the first PCI bus.

> All I am trying to provide right now is having a predictable path for
> CPUs, it doesn't matter if using -device, device_add, -smp, or cpu-add.
> Filtering by device type is not what I need, here.

Ok, so we're on the same page.  I would use any of:

- /machine/cpus/NN (your choice)

- /machine/cpu[NN] (Peter's choice)

- /machine/cpus/cpu[NN] (hybrid, resembles /machine/peripheral-anon or
/machine/unattached more)

I'm not sure if "NN" should be a random progressive number (in that case
you can use cpu[*] to let the QOM core pick the number) or the APIC ID.
 You know the domain better than I do.

> Making the path depend on guest-visible bits that can change depending
> on the architeture or machine would make the path less predictable.

You can still list all children of /machine/cpus.  The disadvantage of
the APIC ID is that IDs may have holes even without doing any
hot-unplug; the advantage is that, from a set of online CPUs in the
guest, you can predict the paths in /machine/cpus.

With cpu[*] instead you can have different contents of /machine/cpus
after for example

   cpu_add 3           # adds /machine/cpus/cpu[2] pointing to CPU 3
   cpu_add 2           # adds /machine/cpus/cpu[3] pointing to CPU 2

vs.

   cpu_add 2           # adds /machine/cpus/cpu[2] pointing to CPU 2
   cpu_add 3           # adds /machine/cpus/cpu[3] pointing to CPU 3


> I have an alternative patch that simply adds a "qom-path" field to
> query-cpus. If we find out that making commitments about QOM paths is
> too hard, I can submit it instead.

I don't think it's too hard, but this alternative patch may also make sense.

Paolo