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

[Pierre Morel]

On 16/11/2017 13:35, Cornelia Huck wrote:
> On Thu, 16 Nov 2017 13:02:26 +0100
> Pierre Morel <pmorel@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> 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
> > > > >  ÂÂÂÂÂÂÂ To compile this driver as a module, choose M here: the
> > > > >  ÂÂÂÂÂÂÂ module will be called vfio_ccw.
> > > > > +config VFIO_AP_MATRIX
> > > > > +ÂÂÂ def_tristate m
> > > > > +ÂÂÂ prompt "Support for Adjunct Processor Matrix device interface"
> > > > > +ÂÂÂ depends on ZCRYPT
> > > > > +ÂÂÂ select VFIO
> > > > > +ÂÂÂ select MDEV
> > > > > +ÂÂÂ select VFIO_MDEV
> > > > > +ÂÂÂ select VFIO_MDEV_DEVICE
> > > > > +ÂÂÂ 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 BÃblingen - Germany