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:

rv64imadc_zifoo_zigoo_zafoo_sbar_scar_zxmbaz_xqux_xrux

“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 scalar accesses are supported in userspace, but they may perform poorly. Misaligned vector accesses are only supported if the Zicclsm extension is supported.

Pointer masking

Support for pointer masking in userspace (the Supm extension) is provided via the PR_SET_TAGGED_ADDR_CTRL and PR_GET_TAGGED_ADDR_CTRL prctl() operations. Pointer masking is disabled by default. To enable it, userspace must call PR_SET_TAGGED_ADDR_CTRL with the PR_PMLEN field set to the number of mask/tag bits needed by the application. PR_PMLEN is interpreted as a lower bound; if the kernel is unable to satisfy the request, the PR_SET_TAGGED_ADDR_CTRL operation will fail. The actual number of tag bits is returned in PR_PMLEN by the PR_GET_TAGGED_ADDR_CTRL operation.

Additionally, when pointer masking is enabled (PR_PMLEN is greater than 0), a tagged address ABI is supported, with the same interface and behavior as documented for AArch64 (AArch64 TAGGED ADDRESS ABI).