Re: [Qemu-devel] [PATCH v1] s390x/pci: Warn when adding PCI devices without the 'zpci' feature

[David Hildenbrand <david@redhat.com>]

On 22.01.19 14:23, Cornelia Huck wrote:
> On Tue, 22 Jan 2019 14:20:27 +0100
> David Hildenbrand <david@redhat.com> wrote:
> 
>> On 22.01.19 14:13, Cornelia Huck wrote:
>>> On Tue, 22 Jan 2019 11:06:46 +0100
>>> David Hildenbrand <david@redhat.com> wrote:
>>>   
>>>> On 22.01.19 10:50, Thomas Huth wrote:  
>>>>> On 2019-01-22 10:41, David Hildenbrand wrote:    
>>>>>> We decided to always create the PCI host bridge, even if 'zpci' is not
>>>>>> enabled (due to migration compatibility).    
>>>>>
>>>>> Couldn't we disable the host bridge for newer machine types, and just
>>>>> create it on the old ones for migration compatibility?    
>>>
>>> I very dimly remember some problems with that approach.
>>>   
>>>>
>>>> I think we can with a compat property. However I somewhat dislike that
>>>> the error/warning will then be "no bus" vs. "zpci CPU feature not
>>>> enabled". Somebody who has no idea about that will think he somehow has
>>>> to create a PCI bus on the QEMU comandline.  
>>>
>>> Agreed, "zpci cpu feature not enabled" gives a much better clue.
>>>   
>>>>
>>>> ... however
>>>>  
>>>>>     
>>>>>> This however right now allows
>>>>>> to add zPCI/PCI devices to a VM although the guest will never actually see
>>>>>> them, confusing people that are using a simple CPU model that has no
>>>>>> 'zpci' enabled - "Why isn't this working" (David Hildenbrand)
>>>>>>
>>>>>> Let's check for 'zpci' and at least print a warning that this will not
>>>>>> work as expected. We could also bail out, however that might break
>>>>>> existing QEMU commandlines.
>>>>>>
>>>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>>>> ---
>>>>>>  hw/s390x/s390-pci-bus.c | 5 +++++
>>>>>>  1 file changed, 5 insertions(+)
>>>>>>
>>>>>> diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
>>>>>> index b86a8bdcd4..e7d4f49611 100644
>>>>>> --- a/hw/s390x/s390-pci-bus.c
>>>>>> +++ b/hw/s390x/s390-pci-bus.c
>>>>>> @@ -863,6 +863,11 @@ static void s390_pcihost_pre_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
>>>>>>  {
>>>>>>      S390pciState *s = S390_PCI_HOST_BRIDGE(hotplug_dev);
>>>>>>  
>>>>>> +    if (!s390_has_feat(S390_FEAT_ZPCI)) {
>>>>>> +        warn_report("Adding PCI or zPCI devices without the 'zpci' CPU feature."
>>>>>> +                    " The guest will not be able to see/use these devices.");
>>>>>> +    }    
>>>>>
>>>>> I think it would be better to bail out. The hotplug clearly can not work
>>>>> in this case, and the warn report might go unnoticed, so blocking the
>>>>> hotplug process is likely better to get the attention of the user.    
>>>>
>>>> ... we could also create the bus but bail out here in case the compat
>>>> property strikes (a.k.a. new QEMO machine type).  
>>>
>>> Now you confused me... why should failing be based on a compat property?
>>>   
>>
>> Otherwise, a QEMU comandline that used to work (which could be created
>> by libvirt) would now fail. Are we ok with that?
>>
> 
> I think we should not fail at all in that case, then. Or only for
> hotplug, not for coldplug.
> 

We could fail on hotplug and warn on coldplug. This would keep existing
setups running.

-- 

Thanks,

David / dhildenb