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

[Christian Borntraeger]

On 22.01.2019 13:52, David Hildenbrand wrote:
> On 22.01.19 13:44, Cornelia Huck wrote:
>> On Tue, 22 Jan 2019 10:41:43 +0100
>> David Hildenbrand <address@hidden> wrote:
>>
>>> We decided to always create the PCI host bridge, even if 'zpci' is not
>>> enabled (due to migration compatibility). 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 <address@hidden>
>>> ---
>>>  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.");
>>> +    }
>>> +
>>>      if (object_dynamic_cast(OBJECT(dev), TYPE_PCI_DEVICE)) {
>>>          PCIDevice *pdev = PCI_DEVICE(dev);
>>>  
>>
>> That's hotplug only, isn't it? IIRC coldplugging already fails?
>>
> 
> No, applies also to coldplugging.

Back then we made this a conscious decision, because removing the bridge 
triggered a
lot of issues regarding migration. And the current behaviour actually is a good
match to the real hardware, there are PCI devices in the system that can not be 
used
by guests. I understand that this is kind of surprising, so I am fine with the 
warn_report
but I do not want to have a hard error right now.