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

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Message-ID: <20220427153924.GZ2125828@nvidia.com>
Date:   Wed, 27 Apr 2022 12:39:24 -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 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