Re: [RFC 05/19] s390/zcrypt: base implementation of AP matrix device driver

[Pierre Morel <pmorel@linux.vnet.ibm.com>]

On 16/11/2017 13:35, Cornelia Huck wrote:
> On Thu, 16 Nov 2017 13:02:26 +0100
> Pierre Morel <pmorel@linux.vnet.ibm.com> wrote:
> 
>> On 14/11/2017 17:37, Tony Krowiak wrote:
>>> On 11/14/2017 07:40 AM, Cornelia Huck wrote:
>>>> On Fri, 13 Oct 2017 13:38:50 -0400
>>>> Tony Krowiak <akrowiak@linux.vnet.ibm.com> wrote:
> 
>>>>> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
>>>>> index 48af970..411c19a 100644
>>>>> --- a/arch/s390/Kconfig
>>>>> +++ b/arch/s390/Kconfig
>>>>> @@ -722,6 +722,19 @@ config VFIO_CCW
>>>>>          To compile this driver as a module, choose M here: the
>>>>>          module will be called vfio_ccw.
>>>>> +config VFIO_AP_MATRIX
>>>>> +    def_tristate m
>>>>> +    prompt "Support for Adjunct Processor Matrix device interface"
>>>>> +    depends on ZCRYPT
>>>>> +    select VFIO
>>>>> +    select MDEV
>>>>> +    select VFIO_MDEV
>>>>> +    select VFIO_MDEV_DEVICE
>>>>> +    select IOMMU_API
>>>> I think the more common pattern is to depend on the VFIO configs
>>>> instead of selecting them.
>>> It's ironic because I originally changed from using 'depends on' and
>>> changed it based on review comments made
>>> on our internal mailing list. I'll go with 'depends on'.
>>
>> Is doing like the others a sufficient good reason?
>> What if the first who did this did not really think about it?
>>
>> When an administrator configure the kernel what does he think?
>>
>> - I want to have AP through AP_VFIO in my guests
>> 	and he get implicitly VFIO
>> or
>> - I want to have VFIO
>> 	and he has to explicitly add AP_VFIO too
>>
>> It seems to me that the first is much more user friendly.
>>
>> Please tell me if I missed something. dependencies? collateral damages?
>> my logic is wrong?
> 
> Using select for anything that's not a simple infrastructure dependency
> may lead into trouble (we've had issues in the past where options tried
> to enable other options but missed dependencies).

Understood, using dependencies is safer against a third party 
introducing a bug that would add a dependency to a member of the list 
but not update our list of selections.

> 
> If a user wants to use vfio-ap, I think it is reasonable to expect them
> to figure out that they need both ap and vfio for that.
> 
> [And config help has gotten much better than it was years ago; it's not
> that hard to figure out what is actually needed.]
> 

OK for Darwin selection for admins, (a gentle Darwin :) I acknowledge) 
and on our side we spare to us running after our disappeared AP VFIO.

Regards,

Pierre



-- 
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany