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

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Dec 13 2021, Harald Freudenberger <freude@linux.ibm.com> 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@redhat.com>
>> ---
>>  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.

I don't think you can drop the kvm reference, as the code in vfio-ap
obviously depends on it...

One possibility is simply blocking autoload of the module in userspace by
default, and only allow it to be loaded automatically when e.g. qemu-kvm
is installed on the system. This is obviously something that needs to be
decided by the distros.

(kvm might actually be autoloaded already, so autoloading vfio-ap would
not really make it worse.)