Re: [PATCH] ocxl: Add get_metadata IOCTL to share OCXL information to userspace

[Frederic Barrat <fbarrat@linux.vnet.ibm.com>]

>>> Yeah, I think metadata will evolve for a while till it settle's down.
>>> Since ocxl_ioctl_get_metadata is exposed via uapi, a newer program
>>> calling an older kernel will never work, since the size of that
>>> struct
>>> will always be larger than what the OS supports and our
>>> copy_to_user()
>>> will fail. The other option is for the user program to try all
>>> possible versions till one succeeds, that is bad as well. I think
>>> there are a few ways around it, if we care about this combination.
>>>
>>> Balbir Singh.
>>>
>>
>> We have a number of reserved members at the end of the struct which can
>> be re-purposed for future information (with a corresponding bump of the
>> version number).
> 
> Good point, agreed


I initially had reservations about using an ioctl command for various 
AFU/context parameters because extensibility is going to be a pain. With 
the current reserved fields and versioning, we're ok for some time 
(version handling will remain a bit of a pain but that's life). I agree 
it helps the library by making things more light weight compared to 
sysfs. But if we need to add parameters in the future, we should keep 
sysfs as an option, especially if it's for rarely used parameters, i.e. 
not something we'll need immediately after an open().

   Fred