Re: [PATCH v3 01/11] riscv: Implement cmpxchg32/64() using Zacas

Samuel Holland
2024-07-19T00:45:44Z

Hi Alex,

On 2024-07-17 1:19 AM, Alexandre Ghiti wrote:
> This adds runtime support for Zacas in cmpxchg operations.
>
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> ---
> arch/riscv/Kconfig | 17 +++++++++++++++++
> arch/riscv/Makefile | 3 +++
> arch/riscv/include/asm/cmpxchg.h | 26 +++++++++++++++++++++++---
> 3 files changed, 43 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 05ccba8ca33a..1caaedec88c7 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -596,6 +596,23 @@ config RISCV_ISA_V_PREEMPTIVE
> preemption. Enabling this config will result in higher memory
> consumption due to the allocation of per-task's kernel Vector context.
>
> +config TOOLCHAIN_HAS_ZACAS
> + bool
> + default y
> + depends on !64BIT || $(cc-option,-mabi=lp64 -march=rv64ima_zacas)
> + depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zacas)
> + depends on AS_HAS_OPTION_ARCH
> +
> +config RISCV_ISA_ZACAS
> + bool "Zacas extension support for atomic CAS"
> + depends on TOOLCHAIN_HAS_ZACAS
> + default y
> + help
> + Enable the use of the Zacas ISA-extension to implement kernel atomic
> + cmpxchg operations when it is detected at boot.
> +
> + If you don't know what to do here, say Y.
> +
> config TOOLCHAIN_HAS_ZBB
> bool
> default y
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index 06de9d365088..9fd13d7a9cc6 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -85,6 +85,9 @@ endif
> # Check if the toolchain supports Zihintpause extension
> riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZIHINTPAUSE) := $(riscv-march-y)_zihintpause
>
> +# Check if the toolchain supports Zacas
> +riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZACAS) := $(riscv-march-y)_zacas
> +
> # Remove F,D,V from isa string for all. Keep extensions between "fd" and "v" by
> # matching non-v and non-multi-letter extensions out with the filter ([^v_]*)
> KBUILD_CFLAGS += -march=$(shell echo $(riscv-march-y) | sed -E 's/(rv32ima|rv64ima)fd([^v_]*)v?/\1\2/')
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index 808b4c78462e..5d38153e2f13 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -9,6 +9,7 @@
> #include <linux/bug.h>
>
> #include <asm/fence.h>
> +#include <asm/alternative.h>
>
> #define __arch_xchg_masked(sc_sfx, prepend, append, r, p, n) \
> ({ \
> @@ -134,21 +135,40 @@
> r = (__typeof__(*(p)))((__retx & __mask) >> __s); \
> })
>
> -#define __arch_cmpxchg(lr_sfx, sc_sfx, prepend, append, r, p, co, o, n) \
> +#define __arch_cmpxchg(lr_sfx, sc_cas_sfx, prepend, append, r, p, co, o, n) \
> ({ \
> + __label__ no_zacas, end; \
> register unsigned int __rc; \
> \
> + if (IS_ENABLED(CONFIG_RISCV_ISA_ZACAS)) { \
> + asm goto(ALTERNATIVE("j %[no_zacas]", "nop", 0, \
> + RISCV_ISA_EXT_ZACAS, 1) \
> + : : : : no_zacas); \
> + \
> + __asm__ __volatile__ ( \
> + prepend \
> + " amocas" sc_cas_sfx " %0, %z2, %1\n" \
> + append \
> + : "+&r" (r), "+A" (*(p)) \
> + : "rJ" (n) \
> + : "memory"); \
> + goto end; \
> + } \
> + \
> +no_zacas: \
> __asm__ __volatile__ ( \
> prepend \
> "0: lr" lr_sfx " %0, %2\n" \
> " bne %0, %z3, 1f\n" \
> - " sc" sc_sfx " %1, %z4, %2\n" \
> + " sc" sc_cas_sfx " %1, %z4, %2\n" \
> " bnez %1, 0b\n" \
> append \

This would probably be a good place to use inline ALTERNATIVE instead of an asm
goto. It saves overall code size, and a jump in the non-Zacas case, at the cost
of 3 nops in the Zacas case. (And all the nops can go after the amocas, where
they will likely be hidden by the amocas latency.)

Regards,
Samuel

> "1:\n" \
> : "=&r" (r), "=&r" (__rc), "+A" (*(p)) \
> : "rJ" (co o), "rJ" (n) \
> : "memory"); \
> + \
> +end:; \
> })
>
> #define _arch_cmpxchg(ptr, old, new, sc_sfx, prepend, append) \
> @@ -156,7 +176,7 @@
> __typeof__(ptr) __ptr = (ptr); \
> __typeof__(*(__ptr)) __old = (old); \
> __typeof__(*(__ptr)) __new = (new); \
> - __typeof__(*(__ptr)) __ret; \
> + __typeof__(*(__ptr)) __ret = (old); \
> \
> switch (sizeof(*__ptr)) { \
> case 1: \