Changing user-mode interfaces

[Rafał Wojdyła <omeg@xxxxxxxxxxxxxxxxxxxxxx>]

Hello,

What is the policy on potentially breaking changes in the user-mode 
interface? I have some additions to the gnttab IOCTL (sharing already 
existing memory in addition to driver-allocated one) which require 
changing the xencontrol interface or adding another version similar to 
how the driver interfaces work.
Additionally I'd like to change how the bookkeeping of gnttab user 
requests is done: currently each IOCTL contains a process-unique ID as a 
part of the input. This ID is managed by xencontrol and that can lead to 
bugs if a process opens more than one xencontrol handle (IDs will be 
duplicated, the driver checks for that but the requests will fail).
I think I was the one who wrote that code in the first place (it has 
been some time... ;), but it seems like we can just remove the request 
IDs and xeniface would track gnttab requests by the user-mode address of 
the shared memory (per-process of course). I don't think it's possible 
to share the same region twice so that should be fine?
So since the "new" IOCTL parameters would get rid of the ID parameter 
(and introduce optional address parameter for memory sharing) I'm 
wondering what's the best way to do the change -- introducing a new 
version is probably the safest, but it would be a bit messy. I wonder 
who else besides us (ITL) even uses this code... :)
Anyway, what's your thoughts on this?

--
Rafał Wojdyła
Invisible Things Lab

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