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

Alexandre Ghiti
2024-07-17T15:19:04Z

Hi drew,

On Wed, Jul 17, 2024 at 5:08 PM Andrew Jones <ajones@ventanamicro.com> wrote:
>
> On Wed, Jul 17, 2024 at 08:19:47AM GMT, 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) \
>
> I'd either not bother renaming sc_sfx or also rename it in _arch_cmpxchg.

I'll rename both then.

>
> > ({ \
> > + __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 \
> > "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); \
>
> Is this just to silence some compiler warnings? Can we point out
> whatever the reason is in the commit message?

CAS expects to find the old value in rd (__ret) to check against the
current value in memory before actually swapping with the new value.

But both you and Andrea were confused by this, I'll make it more explicit.

>
> > \
> > switch (sizeof(*__ptr)) { \
> > case 1: \
> > --
> > 2.39.2
> >
>
> Thanks,
> drew

Thanks,

Alex