Re: [PATCH v6 16/21] vfio-pci/zdev: add open/close device hooks

Matthew Rosato
2022-04-27T14:42:22Z

On 4/27/22 10:04 AM, Jason Gunthorpe wrote:
> On Tue, Apr 26, 2022 at 04:08:37PM -0400, Matthew Rosato wrote:
>
>> +static int vfio_pci_zdev_group_notifier(struct notifier_block *nb,
>> + unsigned long action, void *data)
>> +{
>> + struct zpci_dev *zdev = container_of(nb, struct zpci_dev, nb);
>> + int (*fn)(struct zpci_dev *zdev, struct kvm *kvm);
>> + int rc = NOTIFY_OK;
>> +
>> + if (action == VFIO_GROUP_NOTIFY_SET_KVM) {
>> + if (!zdev)
>> + return NOTIFY_DONE;
>> +
>> + fn = symbol_get(kvm_s390_pci_register_kvm);
>> + if (!fn)
>> + return NOTIFY_DONE;
>> +
>> + if (fn(zdev, (struct kvm *)data))
>> + rc = NOTIFY_BAD;
>> +
>> + symbol_put(kvm_s390_pci_register_kvm);
>
> Is it possible this function can be in statically linked arch code?
>
> Or, actually, is zPCI useful anyhow without kvm ie can you just have a
> direct dependency here?
>

zPCI devices (zpci_dev) exist regardless of whether kvm is configured or
not, and you can e.g. bind the associated PCI device to vfio-pci when
KVM is not configured (or module not loaded) and get the existing
vfio-pci-zdev extensions for that device (extra VFIO_DEVICE_INFO
response data). Making a direct dependency on KVM would remove that;
this was discussed in a prior version because this extra info is not
used today outside of a KVM usecase -- but it could be useful in the
future (or we may have other s390-isms that are not specific to kvm that
need vfio-pci-zdev).

As far as statically linking in arch... The fundamental
(un)registration task being done here -- (dis)associating the guest GISA
with the firmware and thus allowing this particular guest to use
firmware assists (or turning it off when kvm == 0) -- is only relevant
to guest passthrough with kvm and calls a number of different routines
that reside in the kvm module during the (un)registration process.
Without a direct dependency I think a symbol lookup still has to
inevitably happen at some point in the call chain.