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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>
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Conor Dooley 2022-12-05 14:45:26 +00:00 committed by Palmer Dabbelt
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commit f07b2b3f9d
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Documentation/riscv/uabi.rst
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@ -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.