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:

  1. Single-letter extensions come first, in canonical order. The canonical order is "IMAFDQLCBKJTPVH".

  2. All multi-letter extensions will be separated from other extensions by an underscore.

  3. Additional standard extensions (starting with 'Z') will be sorted after single-letter extensions and before any higher-privileged extensions.

  4. 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.

  5. 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.

  6. 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.

  7. 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:


"isa" and "hart isa" lines in /proc/cpuinfo

The "isa" line in /proc/cpuinfo describes the lowest common denominator of RISC-V ISA extensions recognized by the kernel and implemented on all harts. The "hart isa" line, in contrast, describes the set of extensions recognized by the kernel on the particular hart being described, even if those extensions may not be present on all harts in the system.

In both lines, the presence of an extension guarantees only that the hardware has the described capability. Additional kernel support or policy changes may be required before an extension's capability is fully usable by userspace programs. Similarly, for S-mode extensions, presence in one of these lines does not guarantee that the kernel is taking advantage of the extension, or that the feature will be visible in guest VMs managed by this kernel.

Inversely, the absence of an extension in these lines does not necessarily mean the hardware does not support that feature. The running kernel may not recognize the extension, or may have deliberately removed it from the listing.

Misaligned accesses

Misaligned accesses are supported in userspace, but they may perform poorly.