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

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Date:   Thu, 16 Nov 2017 13:35:31 +0100
From:   Cornelia Huck <cohuck@...hat.com>
To:     Pierre Morel <pmorel@...ux.vnet.ibm.com>
Cc:     Tony Krowiak <akrowiak@...ux.vnet.ibm.com>,
        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 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.]