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

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Message-Id: <f1def4fc-49d8-8662-208b-fcea22ee87b0@linux.vnet.ibm.com>
Date:   Thu, 16 Nov 2017 09:25:27 -0500
From:   Tony Krowiak <akrowiak@...ux.vnet.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>,
        Pierre Morel <pmorel@...ux.vnet.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, borntraeger@...ibm.com,
        kwankhede@...dia.com, bjsdjshi@...ux.vnet.ibm.com,
        pbonzini@...hat.com, alex.williamson@...hat.com,
        alifm@...ux.vnet.ibm.com, mjrosato@...ux.vnet.ibm.com,
        qemu-s390x@...gnu.org, jjherne@...ux.vnet.ibm.com,
        thuth@...hat.com, pasic@...ux.vnet.ibm.com
Subject: Re: [RFC 05/19] s390/zcrypt: base implementation of AP matrix device
 driver

On 11/16/2017 07:35 AM, Cornelia Huck wrote:
> On Thu, 16 Nov 2017 13:02:26 +0100
> Pierre Morel <pmorel@...ux.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@...ux.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).
>
> 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.]
Is it sufficient to specify 'depends on ZCRYPT && VFIO_MDEV_DEVICE'
since 'VFIO_MDEV_DEVICE depends on VFIO && VFIO_MDEV' and 'VFIO_MDEV
depends on VFIO' and 'VFIO depends on IOMMU_API'?
>