Re: [PATCH 1/3] VFIO: take reference to the KVM module

Paolo Bonzini
2026-04-10T08:16:27Z

Il gio 9 apr 2026, 20:59 Sean Christopherson <seanjc@google.com> ha scritto:
>
> On Tue, Apr 07, 2026, Paolo Bonzini wrote:
> > VFIO is implicitly taking a reference to the KVM module between
> > vfio_device_get_kvm_safe and vfio_device_put_kvm, thanks to
> > symbol_get and symbol_put.
> >
> > In preparation for removing symbol_get and symbol_put themselves
> > from VFIO, actually store a pointer to the KVM module and use
> > module_get()/module_put() to keep KVM alive.
>
> NAK? :-)
>
> I really don't think we should do this. We're reinventing the wheel, and probably
> doing so poorly. As Jason suggested, the proper way to handle this is to pass
> a "struct file" so that e.g. fops_get() pins kvm.ko for us.

We could get rid of the reference count completely (get_file() as a
replacement for kvm_get_kvm(), get_file_active() as a replacement for
kvm_get_kvm_safe()). struct kvm would need to add a back pointer from
struct kvm to struct file, therefore adding and removing a reference
count would have some additional pointer chasing. KVM has too many
kinds of files to seriously consider passing around struct file* in
virt/kvm/ and arch/*/kvm/, and you'd also have pointer chasing to get
filp->private_data so it wouldn't win much.

Passing both struct kvm and struct file, instead, would be worse
conceptually than this patch (because VFIO doesn't really care about
the fops as opposed to the module) and uglier:
- you can introduce a back pointer to struct kvm
- you can change all struct kvm_device_ops implementations to receive
a struct file*

While I can look at using the file reference for struct kvm, I'm not
sure it's a win overall.

Paolo

>
> https://lore.kernel.org/all/20231203140756.GI1489931@ziepe.ca
>
>