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

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Message-ID: <08458083-5716-0a3b-a4bc-36fc6c136836@linux.ibm.com>
Date:   Thu, 27 Jan 2022 09:23:53 -0500
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     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 12/2/21 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@...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:
>>> 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