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

[Pierre Morel <pmorel@linux.ibm.com>]

On 23/11/2018 13:45, Cornelia Huck wrote:
> On Fri, 23 Nov 2018 13:28:25 +0100
> Pierre Morel <pmorel@linux.ibm.com> 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 <cohuck@redhat.com>
>>> ---
>>>    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