diff options
| author | Conor Dooley <conor.dooley@microchip.com> | 2022-12-05 14:45:26 +0000 |
|---|---|---|
| committer | Palmer Dabbelt <palmer@rivosinc.com> | 2023-01-17 22:05:33 -0800 |
| commit | f07b2b3f9d47fea308af3ae05613b6b4801e68a3 (patch) | |
| tree | 9261f2c931cb0639cc1019dfd1c4ed7a9bd663ba /Documentation/riscv | |
| parent | 80c200b34ee8a0a3378d2073bd8eaae09651c60e (diff) | |
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.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Diffstat (limited to 'Documentation/riscv')
| -rw-r--r-- | Documentation/riscv/uabi.rst | 42 |
1 files changed, 42 insertions, 0 deletions
diff --git a/Documentation/riscv/uabi.rst b/Documentation/riscv/uabi.rst index 21a82cfb6c4d..2ebec4c52230 100644 --- a/Documentation/riscv/uabi.rst +++ b/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. |