Re: [Qemu-devel] [PATCH 1/1] s390x: create a compat s390 phb for <=2.10

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Cornelia Huck (cohuck@redhat.com) wrote:
> On Wed, 27 Sep 2017 15:28:38 +0100
> "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> 
> > * David Hildenbrand (david@redhat.com) wrote:
> > > On 27.09.2017 12:59, Christian Borntraeger wrote:  
> > > > 
> > > > 
> > > > On 09/27/2017 12:56 PM, Cornelia Huck wrote:  
> > > >> On Wed, 27 Sep 2017 18:25:00 +0800
> > > >> Yi Min Zhao <zyimin@linux.vnet.ibm.com> wrote:
> > > >>  
> > > >>> 在 2017/9/27 下午5:47, Cornelia Huck 写道:  
> > > >>>> On Tue, 26 Sep 2017 20:40:25 +0200
> > > >>>> David Hildenbrand <david@redhat.com> wrote:  
> > > >>  
> > > >>>>> I'd really really really (did I mention really?) favor something like a
> > > >>>>> dummy device, because we could easily handle the !CONFIG_PCI case then.
> > > >>>>>
> > > >>>>> All these compat options and conditions will kill us someday... we're
> > > >>>>> already patching around that whole stuff way too much.
> > > >>>>>
> > > >>>>> If we ever unconditionally created a device, we should keep doing so.    
> > > >>>> Yes, that whole thing is horrible, especially interaction with compat
> > > >>>> machines.
> > > >>>>
> > > >>>> Do you have an idea on how to create such a dummy device (without
> > > >>>> having to effectively copy a lot of configured-out code)?
> > > >>>>
> > > >>>>    
> > > >>> How about in s390_pcihost_hot_plug() we check s390_has_feat(zpci)?
> > > >>> If no zpci feature, we avoid plugging any pci device.
> > > >>> Then we could always create phb.
> > > >>> I think pcibus's vmstate is only data to migrate.  
> > > >>
> > > >> That's still problematic if CONFIG_PCI is off. I currently don't have a
> > > >> better idea than either disallowing compat machines on builds without
> > > >> pci, or using a dummy device...  
> > > > 
> > > > For this particular case your initial patch might be less problematic than
> > > > a dummy device, because the code that does the migration is NOT contained
> > > > in s390 specific code but in common PCI code instead. We would need to keep
> > > > the dummy device always in a way that it will work with the common PCI
> > > > code.
> > > >   
> > > 
> > > Interesting, so how is migration then handled for e.g. x86 or other
> > > architectures that can work without CONFIG_PCI? I assume their migration
> > > should also break?  
> > 
> > It's tied to machine-type; the x86 i440fx and q35 machine types have
> > PCI; you can't disable PCI while still having those machine types.
> > (I don't know if you can disable PCI at all on x86)
> 
> Ugh, that sounds like we need two machine types on s390x as well
> (s390x-ccw-virtio and s390x-ccw-virtio-nopci or so), built
> conditionally. That whole zpci detanglement is looking worse and
> worse :(

Well fundamentally the migration expects to migrate something into
the same shaped hole on the destination;  if you've got a lump of PCI
config on the source there's got to be somewhere for it to fit on the
destination.
Now, if PCI is actually pretty rare; then you might be able to make
the host-pci bridge a normal device and not include it in any
machine type; that way those who want PCI can just instantiate
the host-pci bridge, and those who don't want it just stick with
the base machine type.

Dave
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK