Re: [RFC PATCH] s390: vfio-ap: Register the vfio_ap module for the "ap" parent bus

[Thomas Huth]

On 14/12/2021 22.28, Tony Krowiak wrote:
> On 12/13/21 10:44, Harald Freudenberger wrote:
> > On 01.12.21 15:11, Thomas Huth wrote:
> > > The crypto devices that we can use with the vfio_ap module are sitting
> > > on the "ap" bus, not on the "vfio_ap" bus that the module defines
> > > itself. With this change, the vfio_ap module now gets automatically
> > > loaded if a supported crypto adapter is available in the host.
> > >
> > > Signed-off-by: Thomas Huth <thuth@xxxxxxxxxx>
> > > ---
> > >   Note: Marked as "RFC" since I'm not 100% sure about it ...
> > >         please review carefully!
> > >
> > >   drivers/s390/crypto/vfio_ap_drv.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/s390/crypto/vfio_ap_drv.c 
> > > b/drivers/s390/crypto/vfio_ap_drv.c
> > > index 4d2556bc7fe5..5580e40608a4 100644
> > > --- a/drivers/s390/crypto/vfio_ap_drv.c
> > > +++ b/drivers/s390/crypto/vfio_ap_drv.c
> > > @@ -39,7 +39,7 @@ static struct ap_device_id ap_queue_ids[] = {
> > >       { /* end of sibling */ },
> > >   };
> > > -MODULE_DEVICE_TABLE(vfio_ap, ap_queue_ids);
> > > +MODULE_DEVICE_TABLE(ap, ap_queue_ids);
> > >   /**
> > >    * vfio_ap_queue_dev_probe:
> > I had a chance to check this now.
> > First I have to apologize about the dispute with vfio devices appearing on 
> > the ap bus.
> > That's not the case with this patch. As Connie states the 
> > MODULE_DEVICE_TABLE() does not
> > change the parent of a device and vfio_ap_drv is a driver for ap devices 
> > and thus
> > belongs to the ap bus anyway.
> > So what's left is that with this change the vfio_ap kernel module is 
> > automatically loaded
> > when an ap device type 10-13 is recognized by the ap bus. So the intention 
> > of the patch
> > is fulfilled.
> > Yet another kernel module which may occupy memory but will never get used 
> > by most customers.
> > This may not be a problem but I had a glance at the list of kernel modules 
> > loaded on my
> > LPAR with and without the patch and the difference is:
> > ...
> > kvm                   512000  1 vfio_ap
> > vfio_ap                28672  0
> > ...
> > So the vfio_ap module has a dependency to the biggest kernel module ever - 
> > kvm.
> > Do I need to say something more?
> >
> > If this dependency is removed then I would not hesitate to accept this 
> > patch. However
> > this is up to Tony as he is the maintainer of the vfio ap device driver.
>
> The vfio_ap device driver has a dependency on kvm, it can not be removed.
> If the user base for vfio_ap is minimal, then I see no reason why the vfio_ap
> module should be automatically loaded when an AP device type 10-13 is
> recognized by the AP bus. The module is needed only to pass through AP
> queue devices to a KVM guest.

To continue the discussion here - it seems like my patch here won't be 
accepted? Shall I send another one instead to remove the bad 
MODLE_DEVICE_TABLE from the vfio_ap_drv.c file?

 Thomas