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

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 09/27/2017 05:03 PM, Cornelia Huck wrote:
> On Wed, 27 Sep 2017 15:49:27 +0100
> "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> 
>> * 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.
> 
> I fear that ship has already sailed; the s390-ccw-virtio machine type
> has been instantiating a phb for quite some time, which means we have
> to drag this on in the compat machines...

In the end that means that you should revert

commit d32bd032d8fde41281aae34c16a4aa97e9acfeac
Author:     Cornelia Huck <cohuck@redhat.com>
AuthorDate: Thu Jul 6 17:21:52 2017 +0200
Commit:     Cornelia Huck <cohuck@redhat.com>
CommitDate: Wed Aug 30 18:23:25 2017 +0200

    s390x/ccw: create s390 phb conditionally

to keep s390-ccw-virtio clean proper.

If you want to have PCI disabled, you can do you in a machine like 
s390-rhelx.y.z or whatever.