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std_detect: RISC-V platform guide documentation
This is practically a revert of a revert, making the commit e907456b2e10622ccd854a3bba8d02ce170b5dbb on `stdarch` come around again with minor fixes, enhancements and adjustments. An excerpt from the original commit message follows: Since there's no architectural feature detection on RISC-V (unlike `CPUID` on x86 architectures and some system registers on Arm/AArch64), runtime feature detection entirely depends on the platform-specific facility. As a result, availability of each feature heavily depends on the platform and its version. To help users make a decision for feature checking on a RISC-V system, this commit adds a platform guide with minimum supported platform versions.
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‎library/std_detect/src/detect/arch/riscv.rs‎

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@@ -37,90 +37,121 @@ features! {
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///
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/// # Unprivileged Specification
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///
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/// The supported ratified RISC-V instruction sets are as follows:
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/// The supported ratified RISC-V instruction sets are as follows (OS
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/// columns denote runtime feature detection support with or without the
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/// minimum supported version):
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///
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/// * RV32E: `"rv32e"`
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/// * RV32I: `"rv32i"`
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/// * RV64I: `"rv64i"`
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/// * A: `"a"`
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/// * Zaamo: `"zaamo"`
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/// * Zalrsc: `"zalrsc"`
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/// * B: `"b"`
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/// * Zba: `"zba"`
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/// * Zbb: `"zbb"`
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/// * Zbs: `"zbs"`
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/// * C: `"c"`
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/// * Zca: `"zca"`
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/// * Zcd: `"zcd"` (if D is enabled)
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/// * Zcf: `"zcf"` (if F is enabled on RV32)
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/// * D: `"d"`
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/// * F: `"f"`
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/// * M: `"m"`
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/// * Q: `"q"`
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/// * V: `"v"`
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/// * Zve32x: `"zve32x"`
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/// * Zve32f: `"zve32f"`
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/// * Zve64x: `"zve64x"`
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/// * Zve64f: `"zve64f"`
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/// * Zve64d: `"zve64d"`
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/// * Zicbom: `"zicbom"`
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/// * Zicboz: `"zicboz"`
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/// * Zicntr: `"zicntr"`
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/// * Zicond: `"zicond"`
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/// * Zicsr: `"zicsr"`
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/// * Zifencei: `"zifencei"`
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/// * Zihintntl: `"zihintntl"`
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/// * Zihintpause: `"zihintpause"`
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/// * Zihpm: `"zihpm"`
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/// * Zimop: `"zimop"`
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/// * Zabha: `"zabha"`
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/// * Zacas: `"zacas"`
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/// * Zawrs: `"zawrs"`
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/// * Zfa: `"zfa"`
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/// * Zfbfmin: `"zfbfmin"`
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/// * Zfh: `"zfh"`
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/// * Zfhmin: `"zfhmin"`
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/// * Zfinx: `"zfinx"`
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/// * Zdinx: `"zdinx"`
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/// * Zhinx: `"zhinx"`
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/// * Zhinxmin: `"zhinxmin"`
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/// * Zcb: `"zcb"`
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/// * Zcmop: `"zcmop"`
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/// * Zbc: `"zbc"`
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/// * Zbkb: `"zbkb"`
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/// * Zbkc: `"zbkc"`
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/// * Zbkx: `"zbkx"`
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/// * Zk: `"zk"`
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/// * Zkn: `"zkn"`
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/// * Zknd: `"zknd"`
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/// * Zkne: `"zkne"`
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/// * Zknh: `"zknh"`
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/// * Zkr: `"zkr"`
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/// * Zks: `"zks"`
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/// * Zksed: `"zksed"`
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/// * Zksh: `"zksh"`
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/// * Zkt: `"zkt"`
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/// * Zvbb: `"zvbb"`
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/// * Zvbc: `"zvbc"`
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/// * Zvfbfmin: `"zvfbfmin"`
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/// * Zvfbfwma: `"zvfbfwma"`
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/// * Zvfh: `"zvfh"`
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/// * Zvfhmin: `"zvfhmin"`
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/// * Zvkb: `"zvkb"`
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/// * Zvkg: `"zvkg"`
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/// * Zvkn: `"zvkn"`
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/// * Zvkned: `"zvkned"`
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/// * Zvknha: `"zvknha"`
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/// * Zvknhb: `"zvknhb"`
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/// * Zvknc: `"zvknc"`
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/// * Zvkng: `"zvkng"`
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/// * Zvks: `"zvks"`
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/// * Zvksed: `"zvksed"`
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/// * Zvksh: `"zvksh"`
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/// * Zvksc: `"zvksc"`
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/// * Zvksg: `"zvksg"`
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/// * Zvkt: `"zvkt"`
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/// * Ztso: `"ztso"`
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/// | Literal | Base | Linux |
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/// |:---------- |:------- |:---------- |
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/// | `"rv32e"` | RV32E | No |
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/// | `"rv32i"` | RV32I | Yes [^ima] |
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/// | `"rv64i"` | RV64I | Yes [^ima] |
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///
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/// | Literal | Extension | Linux |
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/// |:--------------- |:----------- |:------------------- |
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/// | `"a"` | A | Yes [^ima] |
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/// | `"b"` | B | 6.5 |
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/// | `"c"` | C | Yes |
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/// | `"d"` | D | Yes |
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/// | `"f"` | F | Yes |
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/// | `"m"` | M | Yes [^ima] |
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/// | `"q"` | Q | No |
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/// | `"v"` | V | 6.5 |
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/// | `"zaamo"` | Zaamo | 6.15 [^ima] [^dep] |
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/// | `"zabha"` | Zabha | 6.16 |
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/// | `"zacas"` | Zacas | 6.8 |
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/// | `"zalrsc"` | Zalrsc | 6.15 [^ima] [^dep] |
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/// | `"zawrs"` | Zawrs | 6.11 |
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/// | `"zba"` | Zba | 6.5 |
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/// | `"zbb"` | Zbb | 6.5 |
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/// | `"zbc"` | Zbc | 6.8 |
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/// | `"zbkb"` | Zbkb | 6.8 |
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/// | `"zbkc"` | Zbkc | 6.8 |
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/// | `"zbkx"` | Zbkx | 6.8 |
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/// | `"zbs"` | Zbs | 6.5 |
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/// | `"zca"` | Zca | 6.11 [^dep] |
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/// | `"zcb"` | Zcb | 6.11 |
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/// | `"zcd"` | Zcd | 6.11 [^dep] |
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/// | `"zcf"` | Zcf | 6.11 [^dep] |
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/// | `"zcmop"` | Zcmop | 6.11 |
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/// | `"zdinx"` | Zdinx | No |
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/// | `"zfa"` | Zfa | 6.8 |
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/// | `"zfbfmin"` | Zfbfmin | 6.15 |
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/// | `"zfh"` | Zfh | 6.8 |
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/// | `"zfhmin"` | Zfhmin | 6.8 |
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/// | `"zfinx"` | Zfinx | No |
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/// | `"zhinx"` | Zhinx | No |
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/// | `"zhinxmin"` | Zhinxmin | No |
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/// | `"zicbom"` | Zicbom | 6.15 |
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/// | `"zicboz"` | Zicboz | 6.7 |
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/// | `"zicntr"` | Zicntr | 6.15 [^ima] [^cntr] |
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/// | `"zicond"` | Zicond | 6.8 |
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/// | `"zicsr"` | Zicsr | No [^ima] [^dep] |
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/// | `"zifencei"` | Zifencei | No [^ima] |
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/// | `"zihintntl"` | Zihintntl | 6.8 |
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/// | `"zihintpause"` | Zihintpause | 6.10 |
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/// | `"zihpm"` | Zihpm | 6.15 [^cntr] |
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/// | `"zimop"` | Zimop | 6.11 |
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/// | `"zk"` | Zk | No [^zkr] |
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/// | `"zkn"` | Zkn | 6.8 |
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/// | `"zknd"` | Zknd | 6.8 |
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/// | `"zkne"` | Zkne | 6.8 |
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/// | `"zknh"` | Zknh | 6.8 |
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/// | `"zkr"` | Zkr | No [^zkr] |
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/// | `"zks"` | Zks | 6.8 |
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/// | `"zksed"` | Zksed | 6.8 |
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/// | `"zksh"` | Zksh | 6.8 |
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/// | `"zkt"` | Zkt | 6.8 |
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/// | `"ztso"` | Ztso | 6.8 |
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/// | `"zvbb"` | Zvbb | 6.8 |
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/// | `"zvbc"` | Zvbc | 6.8 |
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/// | `"zve32f"` | Zve32f | 6.11 [^dep] |
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/// | `"zve32x"` | Zve32x | 6.11 [^dep] |
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/// | `"zve64d"` | Zve64d | 6.11 [^dep] |
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/// | `"zve64f"` | Zve64f | 6.11 [^dep] |
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/// | `"zve64x"` | Zve64x | 6.11 [^dep] |
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/// | `"zvfbfmin"` | Zvfbfmin | 6.15 |
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/// | `"zvfbfwma"` | Zvfbfwma | 6.15 |
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/// | `"zvfh"` | Zvfh | 6.8 |
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/// | `"zvfhmin"` | Zvfhmin | 6.8 |
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/// | `"zvkb"` | Zvkb | 6.8 |
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/// | `"zvkg"` | Zvkg | 6.8 |
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/// | `"zvkn"` | Zvkn | 6.8 |
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/// | `"zvknc"` | Zvknc | 6.8 |
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/// | `"zvkned"` | Zvkned | 6.8 |
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/// | `"zvkng"` | Zvkng | 6.8 |
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/// | `"zvknha"` | Zvknha | 6.8 |
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/// | `"zvknhb"` | Zvknhb | 6.8 |
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/// | `"zvks"` | Zvks | 6.8 |
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/// | `"zvksc"` | Zvksc | 6.8 |
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/// | `"zvksed"` | Zvksed | 6.8 |
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/// | `"zvksg"` | Zvksg | 6.8 |
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/// | `"zvksh"` | Zvksh | 6.8 |
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/// | `"zvkt"` | Zvkt | 6.8 |
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///
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/// [^ima]: Or enabled when the IMA base behavior is detected on the Linux
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/// kernel version 6.4 or later (for bases, the only matching one -- either
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/// `"rv32i"` or `"rv64i"` -- is enabled).
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///
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/// [^cntr]: Even if this extension is available, it does not necessarily
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/// mean all performance counters are accessible.
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/// For example, accesses to all performance counters except `time`
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/// (wall-clock) are blocked by default on the Linux kernel
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/// version 6.6 or later.
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/// Also beware that, even if performance counters like `cycle` and
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/// `instret` are accessible, their value can be unreliable (e.g. returning
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/// the constant value) under certain circumstances.
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///
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/// [^dep]: Or enabled as a dependency of another extension (a superset)
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/// even if runtime detection of this feature itself is not supported (as
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/// long as the runtime detection of the superset is supported).
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///
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/// [^zkr]: Linux does not report existence of this extension even if
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/// supported by the hardware mainly because the `seed` CSR on the Zkr
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/// extension (which provides hardware-based randomness) is normally
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/// inaccessible from the user mode.
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/// For the Zk extension features except this CSR, check existence of both
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/// `"zkn"` and `"zkt"` features instead.
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///
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/// There's also bases and extensions marked as standard instruction set,
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/// but they are in frozen or draft state. These instruction sets are also

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