Re: [qemu-s390x] [PATCH 1/3] vfio-ccw: add capabilities chain

[Pierre Morel]

On 23/11/2018 13:45, Cornelia Huck wrote:
> On Fri, 23 Nov 2018 13:28:25 +0100
> Pierre Morel <address@hidden> wrote:
>
> > On 22/11/2018 17:54, Cornelia Huck wrote:
> > > Allow to extend the regions used by vfio-ccw. The first user will be
> > > handling of halt and clear subchannel.
> > >
> > > Signed-off-by: Cornelia Huck <address@hidden>
> > > ---
> > >    drivers/s390/cio/vfio_ccw_ops.c     | 182 ++++++++++++++++++++++++----
> > >    drivers/s390/cio/vfio_ccw_private.h |  38 ++++++
> > >    include/uapi/linux/vfio.h           |   1 +
> > >    3 files changed, 195 insertions(+), 26 deletions(-)
> > >    
> >
> > Halt and clear have no parameters (the sub-channel ID is obviously the
> > one of the mediated device).
> >
> > Isn't adding a new sub-region for the purpose of handling halt and clear
> > superfluous?
> >
> > What is the reason not to use simple ioctls ?
>
> Should it turn out that we missed something and need an enhanced
> interface, we can simply stop providing this subregion and add a new
> subregion, without breaking existing userspace. We can't do that with
> ioctls.

OK, it is a good reason, took me a while but I get the interest of 
capabilities for regions too.

> And moreover, this is only the first user of this infrastructure.
> There's also that path handling series that Dong Jia had posted early
> this year -- that would be an obvious user as well.

right.

Thanks.

Regards,
Pierre

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