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

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Message-ID: <6aaf6c60-a258-29e3-fcec-82c77d3945a4@redhat.com>
Date:   Wed, 15 Dec 2021 13:05:53 +0100
From:   Thomas Huth <thuth@...hat.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Harald Freudenberger <freude@...ux.ibm.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>,
        Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [RFC PATCH] s390: vfio-ap: Register the vfio_ap module for the
 "ap" parent bus

On 14/12/2021 22.55, Tony Krowiak wrote:
> 
> 
> 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.)
> 
> Of the vfio_ccw module is automatically loaded, then the kvm
> module will also get loaded. I startup up a RHEL8.3 system and
> sure enough, the vfio_ccw module is loaded along with the
> kvm, vfio and mdev modules. If this is true for all distros, then
> it wouldn't make much difference if the vfio_ap module is
> autoloaded too.

I think I don't mind too much if we auto-load vfio-ap or not - but I think 
we should make it consistent with vfio-ccw. So either auto-load both modules 
(if the corresponding devices are available), or remove the 
MODULE_DEVICE_TABLE() entries from both modules?

  Thomas