Re: [Qemu-devel] [PATCH RFC 0/2] s390x: cut down on unattached devices

[Halil Pasic <pasic@linux.vnet.ibm.com>]

On 12/07/2017 05:34 PM, Cornelia Huck wrote:
> On Tue, 5 Dec 2017 09:59:06 +0100
> Bjoern Walk <bwalk@linux.vnet.ibm.com> wrote:
> 
>> Cornelia Huck <cohuck@redhat.com> [2017-11-28, 02:46PM +0100]:
>>> info qom-tree shows several devices under unattached that probably
>>> should go somewhere.
>>>
>>> The css bridge should attach to the machine, as it has a similar
>>> purpose as e.g. a pci host bridge.
>>>
>>> The autogenerated network devices should be in the same bucket as any
>>> other device; I'm just not sure about the way I went about it.
>>>
>>> The zpci devices are still problematic: I don't have a good idea where
>>> they should show up.
>>>
>>> Remaining in the unattached container are the sysbus, memory regions
>>> and cpus.
>>>
>>> Cornelia Huck (2):
>>>   s390x/css: attach css bridge
>>>   s390x: attach autogenerated nics
>>>
>>>  hw/s390x/css-bridge.c      | 2 ++
>>>  hw/s390x/s390-virtio-ccw.c | 2 ++
>>>  2 files changed, 4 insertions(+)
>>>
>>> -- 
>>> 2.13.6
>>>
>>>   
>>
>> Regarding the discussion about whether the QOM tree is API and what
>> exploiters like libvirt should do, Halil asked me to chip in.
>>
>> This patch is fine from libvirt perspective. I did a quick smoke test
>> and you can have a
>>
>>     Tested-by: Bjoern Walk <bwalk@linux.vnet.ibm.com>
>>
>> for what it's worth.
> 
> Thanks for checking.
> 
>>
>> In general, I kind of agree with Halil. Unless somewhere in QEMU it is
>> documented that the QOM tree is not guaranteed to be stable for
>> exploiters, I'd consider is part of the API. libvirt does use at least
>> some hardcoded paths, most of the time for CPUs in /machine/unattached,
>> so if that relation would change, things break. However, there is also
>> code to traverse the QOM tree recursively and find a path for a given
>> type(?) name. If this is the preferred way, we probably should change
>> this in libvirt to be safe.
> 
> OK, with that in mind and as we're now adding a property to check on
> the css bridge, I vote for including patch 1 now (having a fixed
> location under /machine looks saner that having to
> check /machine/unattached/device[<n>], which might not be stable).
> 
> Patch 2 needs more discussion, as I'm not sure whether what I'm doing
> is the correct way to go about this (and other machines are in the same
> situation). Not sure whether it is worth trying to attach the zpci
> devices somewhere.
> 

I think, if it's kind of API, then fixing sooner is better than fixing
later.

I also agree that patch 1 should be higher priority.

Before we do patch 1 I would like having agreed and documented whether
this is API or not.

If we decide it's an API, I think we should consider deprecating
the current interface, but keep it working for two releases or
so. I think nothing speaks against introducing a link form unattached
in patch 1 (but I have not tried yet).

Halil