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

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

* 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)

Dave

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