[PATCH RFC] tools/memory-model/README: Expand dependency of klitmus7

[Akira Yokosawa]

klitmus7 is independent of the memory model but depends on the
build-target kernel release.
It occasionally lost compatibility due to kernel API changes [1, 2, 3].
It was remedied in a backwards-compatible manner respectively [4, 5, 6].

Reflect this fact in README.

[1]: b899a850431e ("compiler.h: Remove ACCESS_ONCE()")
[2]: 0bb95f80a38f ("Makefile: Globally enable VLA warning")
[3]: d56c0d45f0e2 ("proc: decouple proc from VFS with "struct proc_ops"")
[4]: https://github.com/herd/herdtools7/commit/e87d7f9287d1
     ("klitmus: Use WRITE_ONCE and READ_ONCE in place of deprecated ACCESS_ONCE")
[5]: https://github.com/herd/herdtools7/commit/a0cbb10d02be
     ("klitmus: Avoid variable length array")
[6]: https://github.com/herd/herdtools7/commit/46b9412d3a58
     ("klitmus: Linux kernel v5.6.x compat")

NOTE: [5] was ahead of herdtools7 7.53, which did not make an
official release.  Code generated by klitmus7 without [5] can still be
built targeting Linux 4.20--5.5 if you don't care VLA warnings.

Signed-off-by: Akira Yokosawa <akiyks@xxxxxxxxx>
---
Hi all,

Recent struggle of Andrii with the use of klitmus7 on an up-to-date
Linux system prompted me to add some explanation of klitmus7's dependency
in README.

As herdtools7 7.56 is still under development, I called out just HEAD
in the compatibility table.  Once 7.56 is released, the table can be
updated.

I'm not sure if this is the right place to carry such info.
Anyway, I'd be glad if this patch can trigger a meaningful update of
README.

        Thanks, Akira
--
 tools/memory-model/README | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/tools/memory-model/README b/tools/memory-model/README
index b9c562e92981..90af203c3cf1 100644
--- a/tools/memory-model/README
+++ b/tools/memory-model/README
@@ -28,8 +28,34 @@ downloaded separately:
 See "herdtools7/INSTALL.md" for installation instructions.
 
 Note that although these tools usually provide backwards compatibility,
-this is not absolutely guaranteed.  Therefore, if a later version does
-not work, please try using the exact version called out above.
+this is not absolutely guaranteed.
+
+For example, a future version of herd7 might not work with the model
+in this release.  A compatible model will likely be made available in
+a later release of Linux kernel.
+
+If you absolutely need to run the model in this particular release,
+please try using the exact version called out above.
+
+klitmus7 is independent of the model provided here.  It has its own
+dependency on a target kernel release where converted code is built
+and executed.  Any change in kernel APIs essential to klitmus7 will
+necessitate an upgrade of klitmus7.
+
+If you find any compatibility issues in klitmus7, please inform the
+memory model maintainers.
+
+klitmus7 Compatibility Table
+----------------------------
+
+	============  ==========
+	target Linux  herdtools7
+	------------  ----------
+	     -- 4.18  7.48 --
+	4.15 -- 4.19  7.49 --
+	4.20 -- 5.5   7.54 --
+	5.6  --       HEAD
+	============  ==========
 
 
 ==================
-- 
2.17.1