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1.. SPDX-License-Identifier: GPL-2.02 3RISC-V Linux User ABI4=====================5 6ISA string ordering in /proc/cpuinfo7------------------------------------8 9The canonical order of ISA extension names in the ISA string is defined in10chapter 27 of the unprivileged specification.11The specification uses vague wording, such as should, when it comes to ordering,12so for our purposes the following rules apply:13 14#. Single-letter extensions come first, in canonical order.15 The canonical order is "IMAFDQLCBKJTPVH".16 17#. All multi-letter extensions will be separated from other extensions by an18 underscore.19 20#. Additional standard extensions (starting with 'Z') will be sorted after21 single-letter extensions and before any higher-privileged extensions.22 23#. For additional standard extensions, the first letter following the 'Z'24 conventionally indicates the most closely related alphabetical25 extension category. If multiple 'Z' extensions are named, they will be26 ordered first by category, in canonical order, as listed above, then27 alphabetically within a category.28 29#. Standard supervisor-level extensions (starting with 'S') will be listed30 after standard unprivileged extensions. If multiple supervisor-level31 extensions are listed, they will be ordered alphabetically.32 33#. Standard machine-level extensions (starting with 'Zxm') will be listed34 after any lower-privileged, standard extensions. If multiple machine-level35 extensions are listed, they will be ordered alphabetically.36 37#. Non-standard extensions (starting with 'X') will be listed after all standard38 extensions. If multiple non-standard extensions are listed, they will be39 ordered alphabetically.40 41An example string following the order is::42 43 rv64imadc_zifoo_zigoo_zafoo_sbar_scar_zxmbaz_xqux_xrux44 45"isa" and "hart isa" lines in /proc/cpuinfo46-------------------------------------------47 48The "isa" line in /proc/cpuinfo describes the lowest common denominator of49RISC-V ISA extensions recognized by the kernel and implemented on all harts. The50"hart isa" line, in contrast, describes the set of extensions recognized by the51kernel on the particular hart being described, even if those extensions may not52be present on all harts in the system.53 54In both lines, the presence of an extension guarantees only that the hardware55has the described capability. Additional kernel support or policy changes may be56required before an extension's capability is fully usable by userspace programs.57Similarly, for S-mode extensions, presence in one of these lines does not58guarantee that the kernel is taking advantage of the extension, or that the59feature will be visible in guest VMs managed by this kernel.60 61Inversely, the absence of an extension in these lines does not necessarily mean62the hardware does not support that feature. The running kernel may not recognize63the extension, or may have deliberately removed it from the listing.64 65Misaligned accesses66-------------------67 68Misaligned scalar accesses are supported in userspace, but they may perform69poorly. Misaligned vector accesses are only supported if the Zicclsm extension70is supported.71