From 8f501be87e45112eff74d0569dcfaab6bce39ef5 Mon Sep 17 00:00:00 2001 From: Tsukasa OI Date: Wed, 26 Jul 2023 05:44:16 +0000 Subject: [PATCH] RISC-V: clarify the QEMU workaround in ISA parser Extensions prefixed with "Su" won't corrupt the workaround in many cases. The only exception is when the first multi-letter extension in the ISA string begins with "Su" and is not prefixed with an underscore. For instance, following ISA string can confuse this QEMU workaround. * "rv64imacsuclic" (RV64I + M + A + C + "Suclic") However, this case is very unlikely because extensions prefixed by either "Z", "Sm" or "Ss" will most likely precede first. For instance, the "Suclic" extension (draft as of now) will be placed after related "Smclic" and "Ssclic" extensions. It's also highly likely that other unprivileged extensions like "Zba" will precede. It's also possible to suppress the issue in the QEMU workaround with an underscore. Following ISA string won't confuse the QEMU workaround. * "rv64imac_suclic" (RV64I + M + A + C + delimited "Suclic") This fix is to tell kernel developers the nature of this workaround precisely. There are some "Su*" extensions to be ratified but don't worry about this workaround too much. This commit comes with other minor editorial fixes (for minor wording and spacing issues, without changing the meaning). Signed-off-by: Tsukasa OI Reviewed-by: Conor Dooley Link: https://lore.kernel.org/r/8a127608cf6194a6d288289f2520bd1744b81437.1690350252.git.research_trasio@irq.a4lg.com Signed-off-by: Palmer Dabbelt --- arch/riscv/kernel/cpufeature.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c index 8ad6da03ee348..9e0d375a40f6e 100644 --- a/arch/riscv/kernel/cpufeature.c +++ b/arch/riscv/kernel/cpufeature.c @@ -204,10 +204,11 @@ static void __init riscv_parse_isa_string(unsigned long *this_hwcap, struct risc switch (*ext) { case 's': /* - * Workaround for invalid single-letter 's' & 'u'(QEMU). + * Workaround for invalid single-letter 's' & 'u' (QEMU). * No need to set the bit in riscv_isa as 's' & 'u' are - * not valid ISA extensions. It works until multi-letter - * extension starting with "Su" appears. + * not valid ISA extensions. It works unless the first + * multi-letter extension in the ISA string begins with + * "Su" and is not prefixed with an underscore. */ if (ext[-1] != '_' && ext[1] == 'u') { ++isa; -- 2.30.2