Re: [PATCH v3 05/14] s390: vfio-ap: base implementation of VFIO AP device driver
From: Cornelia Huck
Date: Tue Apr 03 2018 - 05:56:23 EST
On Tue, 27 Mar 2018 16:45:02 +0200
Pierre Morel <pmorel@xxxxxxxxxxxxxxxxxx> wrote:
> On 27/03/2018 13:17, Cornelia Huck wrote:
> > On Thu, 15 Mar 2018 13:25:25 -0400
> > Tony Krowiak <akrowiak@xxxxxxxxxxxxxxxxxx> wrote:
> >> On 03/15/2018 09:25 AM, Pierre Morel wrote:
> >>> On 14/03/2018 19:25, Tony Krowiak wrote:
> >>>> +config VFIO_AP
> >>>> + def_tristate m
> >>> not sure it must be module by default.
> >>> I would not set it by default.
> >> Connie also asked about this in the last review, so I will go ahead
> >> and change it.
> >>>> + prompt "VFIO support for AP devices"
> >>>> + depends on ZCRYPT && VFIO_MDEV_DEVICE
> >>> VFIO_MDEV_DEVICE is a general feature *needed* by VFIO_AP
> >>> and has no use case by its own. If it is set it is obviously because some
> >>> mediated device drivers needs it.
> >>> while ZCRYPT is a Z feature which may be set without VFIO_AP.
> >>> So you need:
> >>> config VFIO_AP
> >>> def_tristate n
> >>> prompt "VFIO support for AP devices"
> >>> depends on ZCRYPT
> >>> select VFIO_MDEV
> >>> select VFIO_MDEV_DEVICE
> >>> ...
> >> I was thinking the same just yesterday and I agree, this makes sense.
> > OTOH, nobody else seems to do a select on these symbols so far.
> > If you decide to go that route, you'll also need to depend on VFIO
> I think a select is better (again).
> > (otherwise you could end up selecting symbols with unmet dependencies).
> > All in all, I prefer the 'depends' approach.
> Why do you prefer this approach?
Hm, I thought I had already written a mail, but apparently I didn't....
> I can tell you why I prefer a mixed approach:
> We have two tools, depends and select.
> It seems to me that depends should be used for things we can not choose
> to be there or not, but things that just are there, like hardware
> dependencies. For example MMU, CPU type, CRYPTO hardware...
> Select on the other hand is useful to choose things that we need like
> libraries, VFIO, VIRTIO, crypto libraries etc.
> Using this policy is clear and makes easy to choose functionalities and
> get the utilities automatically.
> On the other hand, only using depends makes things to hide the
> functionalities behind the utilities.
My view is the following:
- select is useful for library functionality or for enabling
architecture-specific optimizations (the HAVE_xxx symbols),
especially things you don't want the user to deal with. If you select
something, you need to take care of any dependencies yourself.
- depends is useful for more complex dependencies, and especially
things you don't want automagically enabled. [In modern menuconfig,
it is easy to figure out any missing dependencies for a config option
The mdev infrastructure is too complex to be considered a simple
library IMO (cf. the missing VFIO dependency).