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 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 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.

... 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).

Thanks!

> 
>  Thomas
> 


-- 

Thanks,

David / dhildenb