Re: [Qemu-devel] [qemu-s390x] [PATCH v2 3/6] s390x/pci: Warn when adding PCI devices without the 'zpci' feature

[David Hildenbrand <david@redhat.com>]

On 04.02.19 23:42, Collin Walling wrote:
> On 2/4/19 4:54 PM, David Hildenbrand wrote:
>> On 04.02.19 21:19, Collin Walling wrote:
>>> On 1/30/19 10:57 AM, David Hildenbrand 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.
>>>>
>>>> Reviewed-by: Thomas Huth <thuth@redhat.com>
>>>> 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 9b5c5fff60..2efd9186c2 100644
>>>> --- a/hw/s390x/s390-pci-bus.c
>>>> +++ b/hw/s390x/s390-pci-bus.c
>>>> @@ -826,6 +826,11 @@ 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("PCI/zPCI device without the 'zpci' CPU feature."
>>>> +                    " The guest will not be able to see/use this device");
>>>> +    }
>>>> +
>>>>        if (object_dynamic_cast(OBJECT(dev), TYPE_PCI_DEVICE)) {
>>>>            PCIDevice *pdev = PCI_DEVICE(dev);
>>>>    
>>>>
>>>
>>> I wonder if someone might misconstrue this as "the _PCI device_ needs
>>> the zpci feature." I think "'zpci' CPU feature required to support
>>> PCI/zPCI devices." reads better. The last sentence is fine to me.
>>>
>>
>> Well, the guest needs the 'zpci' feature to see the device. And that's
>> what that message says in my opinion. Not that a device needs to have a
>> feature (I added "CPU feature" for this reason).
>>
>> "required to support" does it not make very clear what we actually want
>> to say.
>>
>> Thanks!
>>
> 
> I see your point. We can still plug in the device without the CPU
> feature, but the device will ultimately be useless to the guest. Thanks
> for clearing that up.
> 
> Still, the wording reads strangely to me. I read it as the PCI device
> itself requires a "zpci CPU feature" which of course does not make sense
> (and I fully understand that's not what you mean here).
> 
> What do you think about:
> 
> "PCI/zPCI device plugging without 'zpci' CPU feature enabled." along
> with your second sentence, of course.

"Plugging a PCI/zPCI device without the 'zpci' CPU feature enabled. The
guest will not be able to see/use this device."

would make sense to me!

> 
> Either way you decide, it's still a good idea to have this warning in
> here. I'm really just debating syntax and not semantics, so it's not
> really important. I won't impede this patch over a differing opinion of
> a small rewording. :)
> 
> Reviewed-by: Collin Walling <walling@linux.ibm.com>
> 

Thanks!

-- 

Thanks,

David / dhildenb