linux-kernel - Re: [RFC PATCH] s390: vfio-ap: Register the vfio

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Date:   Thu, 27 Jan 2022 09:46:41 -0500
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>,
        Harald Freudenberger <freude@...ux.ibm.com>,
        Thomas Huth <thuth@...hat.com>, linux-s390@...r.kernel.org,
        Halil Pasic <pasic@...ux.ibm.com>,
        Jason Herne <jjherne@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [RFC PATCH] s390: vfio-ap: Register the vfio_ap module for the
 "ap" parent bus



On 12/13/21 11:11, Cornelia Huck wrote:
> On Mon, Dec 13 2021, Harald Freudenberger <freude@...ux.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@...hat.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.)

The vfio_ccw module, which is automatically loaded, also requires the 
kvm module,
so autoloading vfio_ap will not make much difference.

>