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

Jason Gunthorpe
2022-04-27T15:39:37Z

On Wed, Apr 27, 2022 at 11:26:40AM -0400, Matthew Rosato wrote:
> > > 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.

That make sense to me, I'd rather see that then the symbol_get/put here

Thanks,
Jason