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

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 7 Dec 2017 18:15:57 +0100
Halil Pasic <pasic@linux.vnet.ibm.com> wrote:

> On 12/07/2017 06:06 PM, Cornelia Huck wrote:
> > On Thu, 7 Dec 2017 18:01:46 +0100  
> [..]
> >>>> 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).  
> > 
> > No, just no. That's completely overengineered.
> >   
> 
> Which part is totally overengineered? Having it clear what is API and
> what not? Having this documented? Or caring about our deprecation
> policy (if it's API)?
> 

You're building a monster to fix a non-existing problem. I will not go
down that rabbit hole any further, and just apply patch 1.