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

Matthew Rosato
2022-04-27T15:26:58Z

On 4/27/22 11:01 AM, Jason Gunthorpe wrote:
> On Wed, Apr 27, 2022 at 10:42:07AM -0400, Matthew Rosato wrote:
>> 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
>> are not specific to kvm that need vfio-pci-zdev).
>
> I'm a bit confused, what is the drawback of just having a direct
> symbol dependency here? It means vfio loads a little extra kernel
> module code, but is that really a big worry given almost all vfio
> users on s390 will be using it with kvm?

It's about trying to avoid loading unnecessary code (or at least giving
a way to turn it off).

Previously I did something like....

https://lore.kernel.org/kvm/20220204211536.321475-15-mjrosato@linux.ibm.com/

And could do so again; as discussed in the thread there, I can use e.g.
CONFIG_VFIO_PCI_ZDEV_KVM and make vfio-pci-zdev depend on KVM in this
series. You only get the vfio-pci-zdev extensions when you configure KVM.

Then if we find a usecase for a vfio-pci-zdev extension without KVM
later, we can further split vfio-pci-zdev and go back to building
vfio-pci-zdev when CONFIG_S390 but only build the kvm pieces (like the
code above) when you specify CONFIG_VFIO_PCI_ZDEV_KVM. This would
eliminate this symbol_get. Sound OK?

>
> Or is there some technical blocker? (circular dep or something?)
>
> Jason