1.. SPDX-License-Identifier: GPL-2.0 2 3RISC-V Linux User ABI 4===================== 5 6ISA string ordering in /proc/cpuinfo 7------------------------------------ 8 9The canonical order of ISA extension names in the ISA string is defined in 10chapter 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 an 18 underscore. 19 20#. Additional standard extensions (starting with 'Z') will be sorted after 21 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 alphabetical 25 extension category. If multiple 'Z' extensions are named, they will be 26 ordered first by category, in canonical order, as listed above, then 27 alphabetically within a category. 28 29#. Standard supervisor-level extensions (starting with 'S') will be listed 30 after standard unprivileged extensions. If multiple supervisor-level 31 extensions are listed, they will be ordered alphabetically. 32 33#. Standard machine-level extensions (starting with 'Zxm') will be listed 34 after any lower-privileged, standard extensions. If multiple machine-level 35 extensions are listed, they will be ordered alphabetically. 36 37#. Non-standard extensions (starting with 'X') will be listed after all standard 38 extensions. If multiple non-standard extensions are listed, they will be 39 ordered alphabetically. 40 41An example string following the order is:: 42 43 rv64imadc_zifoo_zigoo_zafoo_sbar_scar_zxmbaz_xqux_xrux 44 45"isa" and "hart isa" lines in /proc/cpuinfo 46------------------------------------------- 47 48The "isa" line in /proc/cpuinfo describes the lowest common denominator of 49RISC-V ISA extensions recognized by the kernel and implemented on all harts. The 50"hart isa" line, in contrast, describes the set of extensions recognized by the 51kernel on the particular hart being described, even if those extensions may not 52be present on all harts in the system. 53 54In both lines, the presence of an extension guarantees only that the hardware 55has the described capability. Additional kernel support or policy changes may be 56required before an extension's capability is fully usable by userspace programs. 57Similarly, for S-mode extensions, presence in one of these lines does not 58guarantee that the kernel is taking advantage of the extension, or that the 59feature will be visible in guest VMs managed by this kernel. 60 61Inversely, the absence of an extension in these lines does not necessarily mean 62the hardware does not support that feature. The running kernel may not recognize 63the extension, or may have deliberately removed it from the listing. 64 65Misaligned accesses 66------------------- 67 68Misaligned scalar accesses are supported in userspace, but they may perform 69poorly. Misaligned vector accesses are only supported if the Zicclsm extension 70is supported. 71 72Pointer masking 73--------------- 74 75Support for pointer masking in userspace (the Supm extension) is provided via 76the ``PR_SET_TAGGED_ADDR_CTRL`` and ``PR_GET_TAGGED_ADDR_CTRL`` ``prctl()`` 77operations. Pointer masking is disabled by default. To enable it, userspace 78must call ``PR_SET_TAGGED_ADDR_CTRL`` with the ``PR_PMLEN`` field set to the 79number of mask/tag bits needed by the application. ``PR_PMLEN`` is interpreted 80as a lower bound; if the kernel is unable to satisfy the request, the 81``PR_SET_TAGGED_ADDR_CTRL`` operation will fail. The actual number of tag bits 82is returned in ``PR_PMLEN`` by the ``PR_GET_TAGGED_ADDR_CTRL`` operation. 83 84Additionally, when pointer masking is enabled (``PR_PMLEN`` is greater than 0), 85a tagged address ABI is supported, with the same interface and behavior as 86documented for AArch64 (Documentation/arch/arm64/tagged-address-abi.rst). 87