Re: [PATCH v3 08/12] riscv: p8700: Add software emulation for AMO* instructions

[Yao Zi <ziyao@disroot.org>]

On Tue, Jul 29, 2025 at 04:24:17PM +0000, Uros Stajic wrote:
> From: Chao-ying Fu <cfu@mips.com>
> 
> This patch adds software emulation for atomic memory operations (AMO)
> instructions that may not be supported in hardware.
> 
> The `emu-amo.s` file provides assembly implementations of the
> aforementioned operations. Corresponding handler logic is integrated
> into the illegal instruction trap to catch and emulate unsupported
> AMO* instructions at runtime.

The main body of U-Boot is executed by only one CPU, so I don't think
there're chances for atomic instructions; disassembly of a U-Boot binary
built with qemu-riscv64_smode_defconfig confirms the idea: the only four
amo instructions locate in arch/riscv/cpu/start.S for picking the HART
to run U-Boot.

I suggest implementing a LR/SC-based codepath for HART lottery logic in
start.S, and introducing a new Kconfig entry (maybe CONFIG_ISA_ZAMMO) to
represent the availability of Zammo extension. We could use the
alternative path for RISC-V platforms that don't implement Zammo and
disable CONFIG_ISA_ZAAMO, which saves a lot of lines and make the code
cleaner.

Regards,
Yao Zi

> Signed-off-by: Chao-ying Fu <cfu@mips.com>
> Signed-off-by: Uros Stajic <uros.stajic@htecgroup.com>
> ---
>  arch/riscv/cpu/p8700/Makefile  |   1 +
>  arch/riscv/cpu/p8700/emu-amo.S | 254 ++++++++++++++++++++++++++++
>  arch/riscv/lib/interrupts.c    | 299 +++++++++++++++++++++++++++++++++
>  include/interrupt.h            |  19 +++
>  4 files changed, 573 insertions(+)
>  create mode 100644 arch/riscv/cpu/p8700/emu-amo.S