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

[Christian Borntraeger]

On 22.01.2019 16:11, David Hildenbrand wrote:
> On 22.01.19 16:03, Christian Borntraeger wrote:
>>
>>
>> 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.
>>
> 
> So you agree to this patch, unmodified, correct? Thanks!

Sorry, somehow lost this mail. Yes.