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

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Date:   Wed, 27 Apr 2022 11:26:40 -0400
From:   Matthew Rosato <mjrosato@...ux.ibm.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     linux-s390@...r.kernel.org, alex.williamson@...hat.com,
        cohuck@...hat.com, schnelle@...ux.ibm.com, farman@...ux.ibm.com,
        pmorel@...ux.ibm.com, borntraeger@...ux.ibm.com, hca@...ux.ibm.com,
        gor@...ux.ibm.com, gerald.schaefer@...ux.ibm.com,
        agordeev@...ux.ibm.com, svens@...ux.ibm.com, frankja@...ux.ibm.com,
        david@...hat.com, imbrenda@...ux.ibm.com, vneethv@...ux.ibm.com,
        oberpar@...ux.ibm.com, freude@...ux.ibm.com, thuth@...hat.com,
        pasic@...ux.ibm.com, pbonzini@...hat.com, corbet@....net,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v6 16/21] vfio-pci/zdev: add open/close device hooks

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