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

[Tony Krowiak]

On 12/2/21 03:33, Harald Freudenberger wrote:
> On 02.12.21 08:13, Thomas Huth wrote:
> > On 01/12/2021 18.10, 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:
> > > Hello Thomas, interesting.
> > > Did you test this ? I mean did you build a kernel and have it run on a s390 with crypto cards available ?
> > Yes, I've tested it. Without the patch, the vfio_ap module does not get loaded automatically if a crypto card is available. With the patch applied, the vfio_ap module correctly gets loaded automatically on my system (similar to the vfio_ccw module).
> >
> > > My strong feeling is that this will make the AP bus code stumble as the code silently assumes there are exact
> > > two types of ap devices attached to the ap bus: ap cards and ap queues.
> > This is only about getting the module loaded automatically once such a device is available ... AFAIK it does not grab any of the devices automatically, so there shouldn't be any problems?
> >
> >   Thomas
> Yes, of course for the automatic module load works this way. But you understand that now
> the vfio devices are childs of the ap bus and thus are siblings of the ap queue and ap card
> devices. As I wrote the ap bus code is not prepared to deal with a 3th type of devices
> dangling on the ap bus. So you should test what happens when there are real vfio ap devices
> in use together with 'regular' ap card and queue devices.
>
> However, I am still not sure if it is preferable to have the vfio ap module loaded automatically. The majority
> of customers will never use vfio ap devices - this is specific to kvm hosts only. I think this has to be
> decided by Tony and maybe some kvm architect. If there is an agreement, I will try to rework the
> ap code to be able to deal with foreign devices attached to the ap bus.

Can you please explain how foreign devices can get attached to the bus 
if the vfio_ap device
driver is automatically loaded because the MODULE_DEVICE_TABLE 
specification is changed?

Regards,
Tony Krowiak

> So thanks for your investigations. Let's wait for Tony and see how we proceed.
>
> Harald
> Harald