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

[Christian Borntraeger]

On 09/28/2017 12:34 PM, Christian Borntraeger wrote:
> 
> 
> On 09/27/2017 05:03 PM, Cornelia Huck wrote:
>> On Wed, 27 Sep 2017 15:49:27 +0100
>> "Dr. David Alan Gilbert" <address@hidden> wrote:
>>
>>> * Cornelia Huck (address@hidden) wrote:
>>>> On Wed, 27 Sep 2017 15:28:38 +0100
>>>> "Dr. David Alan Gilbert" <address@hidden> wrote:
>>>>   
>>>>> * David Hildenbrand (address@hidden) 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 <address@hidden> wrote:
>>>>>>>>    
>>>>>>>>> 在 2017/9/27 下午5:47, Cornelia Huck 写道:    
>>>>>>>>>> On Tue, 26 Sep 2017 20:40:25 +0200
>>>>>>>>>> David Hildenbrand <address@hidden> 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 <address@hidden>
> AuthorDate: Thu Jul 6 17:21:52 2017 +0200
> Commit:     Cornelia Huck <address@hidden>
> 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 
too quick                                     ^that^                            
         
> s390-rhelx.y.z or whatever. 

I think we really must revert this commit.