summaryrefslogtreecommitdiffstats
path: root/Documentation/riscv/uabi.rst
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/riscv/uabi.rst')
-rw-r--r--Documentation/riscv/uabi.rst48
1 files changed, 48 insertions, 0 deletions
diff --git a/Documentation/riscv/uabi.rst b/Documentation/riscv/uabi.rst
new file mode 100644
index 0000000000..8960fac42c
--- /dev/null
+++ b/Documentation/riscv/uabi.rst
@@ -0,0 +1,48 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+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.