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

From: Halil Pasic
Subject: Re: [qemu-s390x] [Qemu-devel] [PATCH RFC 0/2] s390x: cut down on unattached devices
Date: Thu, 7 Dec 2017 18:01:46 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 12/07/2017 05:34 PM, Cornelia Huck wrote:
> On Tue, 5 Dec 2017 09:59:06 +0100
> Bjoern Walk <address@hidden> wrote:
>> Cornelia Huck <address@hidden> [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 <address@hidden>
>> 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

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).


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