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gcclinux-kernelcompilationcpu-architectureriscv

What CPU instruction sets extensions are needed to support the target 'riscv32' for Linux/GCC?

I was looking into Linux support for the RISC-V 32 Bit and came across the following compile instructions:

make ARCH=riscv CROSS_COMPILE=riscv32-unknown-linux-gnu- -j $(nproc)

The issue is that riscv32 does not make it clear if just the base CPU instruction set is needed (RV32I) or if additional extensions are needed/toggleable (RV32IMAC seems to be a common target).

This leaves me with the following questions:

  • What CPU instruction set or sets is implied in above command?
  • If not RV32I, can optional arguments be added to support RV32I?
Solution

  • From what I can see in the Linux kernel v5.19 the Makefile for RISC-V (arch/riscv/Makefile) looks like this:

    # ISA string setting
riscv-march-$(CONFIG_ARCH_RV32I)    := rv32ima
riscv-march-$(CONFIG_ARCH_RV64I)    := rv64ima
riscv-march-$(CONFIG_FPU)           := $(riscv-march-y)fd
riscv-march-$(CONFIG_RISCV_ISA_C)   := $(riscv-march-y)c

    The ISA used is rv32ima for 32-bit RISC-V kernels. Additionally, the f, d and c extensions can also be enabled by configuring CONFIG_FPU (for fd) and CONFIG_RISCV_ISA_C (for c). By default these two are both set to y so you have rv32imafdc, however you can disable fdc if you run menuconfig (or similar) and set CONFIG_FPU=n and CONFIG_RISCV_ISA_C=n.

    Note: this is for Linux v5.19. You will have to make sure yourself by inspecting the Makefile of your kernel if you have a different version.

    can optional arguments be added to support RV32I?

    No, it does not look like so, rv32ima is the minimum set of extensions selected when tarteging RISC-V 32bit. But again, if you have a different kernel version lower than v5.19 you'd have to check your Makefile to make sure (maybe some older kernels are fine with just rv32i).