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

[Collin Walling <walling@linux.ibm.com>]

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.