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

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Date:   Wed, 27 Apr 2022 12:01:38 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.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 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?

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

Jason