Documentation: riscv: add a section about ISA string ordering in /proc/cpuinfo

The RISC-V specs are permissive in what they allow as the ISA string,
but how we output this to userspace in /proc/cpuinfo is quasi uABI.

Formalise this as part of the uABI, by documenting the list of rules
we use at this point in time.
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20221205144525.2148448-4-conor.dooley@microchip.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/riscv/uabi.rst

@@ -3,4 +3,46 @@
 RISC-V Linux User ABI
 =====================
 
+ISA string ordering in /proc/cpuinfo
+------------------------------------
+
+The canonical order of ISA extension names in the ISA string is defined in
+chapter 27 of the unprivileged specification.
+The specification uses vague wording, such as should, when it comes to ordering,
+so for our purposes the following rules apply:
+
+#. Single-letter extensions come first, in canonical order.
+   The canonical order is "IMAFDQLCBKJTPVH".
+
+#. All multi-letter extensions will be separated from other extensions by an
+   underscore.
+
+#. Additional standard extensions (starting with 'Z') will be sorted after
+   single-letter extensions and before any higher-privileged extensions.
+
+#. For additional standard extensions, the first letter following the 'Z'
+  conventionally indicates the most closely related alphabetical
+  extension category. If multiple 'Z' extensions are named, they will be ordered
+  first by category, in canonical order, as listed above, then alphabetically
+  within a category.
+
+#. Standard supervisor-level extensions (starting with 'S') will be listed
+   after standard unprivileged extensions.  If multiple supervisor-level
+   extensions are listed, they will be ordered alphabetically.
+
+#. Standard machine-level extensions (starting with 'Zxm') will be listed
+   after any lower-privileged, standard extensions. If multiple machine-level
+   extensions are listed, they will be ordered alphabetically.
+
+#. Non-standard extensions (starting with 'X') will be listed after all standard
+   extensions. If multiple non-standard extensions are listed, they will be
+   ordered alphabetically.
+
+An example string following the order is::
+
+   rv64imadc_zifoo_zigoo_zafoo_sbar_scar_zxmbaz_xqux_xrux
+
+Misaligned accesses
+-------------------
+
 Misaligned accesses are supported in userspace, but they may perform poorly.