On Thu, Mar 11, 2021 at 02:55:34PM -0800, Jacob Pan wrote:

Hi Jason,

Thanks for the review.

On Wed, 10 Mar 2021 15:23:01 -0400, Jason Gunthorpe [off-list ref] wrote:

quoted

On Sat, Feb 27, 2021 at 02:01:26PM -0800, Jacob Pan wrote:

quoted

+/* -------- IOCTLs for IOASID file descriptor (/dev/ioasid) -------- */
+
+/**
+ * IOASID_GET_API_VERSION - _IO(IOASID_TYPE, IOASID_BASE + 0)
+ *
+ * Report the version of the IOASID API.  This allows us to bump the
entire
+ * API version should we later need to add or change features in
incompatible
+ * ways.
+ * Return: IOASID_API_VERSION
+ * Availability: Always
+ */
+#define IOASID_GET_API_VERSION		_IO(IOASID_TYPE,
IOASID_BASE + 0)  

I think this is generally a bad idea, if you change the API later then
also change the ioctl numbers and everything should work out

eg use the 4th argument to IOC to specify something about the ABI

Let me try to understand the idea, do you mean something like this?
#define IOASID_GET_INFO _IOC(_IOC_NONE, IOASID_TYPE, IOASID_BASE + 1,
sizeof(struct ioasid_info))

If we later change the size of struct ioasid_info, IOASID_GET_INFO would be
a different ioctl number. Then we will break the existing user space that
uses the old number. So I am guessing you meant we need to have a different
name also. i.e.

Something like that is more appropriate. Generally we should not be
planning to 'remove' IOCTLs. The kernel must always have backwards
compat, so any new format you introduce down the road has to have new
IOCTL number so the old format can continue to be supported.

Negotiation of support can usually by done by probing for ENOIOCTLCMD
or similar on the new ioctls, not an API version

Jason