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

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 4 Feb 2019 23:45:35 +0100
David Hildenbrand <david@redhat.com> wrote:

> 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!

Ok, I have now

        warn_report("Plugging a PCI/zPCI device without the 'zpci' CPU "
                    "feature enabled; the guest will not be able to see/use "
                    "this device");

> 
> > 
> > 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!
>