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

[David Hildenbrand]

On 22.01.19 14:13, Cornelia Huck wrote:
> On Tue, 22 Jan 2019 11:06:46 +0100
> David Hildenbrand <address@hidden> 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 <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.");
>>>> +    }  
>>>
>>> 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?

-- 

Thanks,

David / dhildenb