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

[Alexandre Ghiti]

On Fri, Jul 19, 2024 at 2:45 AM Samuel Holland
<samuel.holland@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxx>
> > ---
> >  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.)

That's what Conor proposed indeed.

I have just given it a try, but it does not work. The number of
instructions in the zacas asm inline is different in the case of the
fully-ordered version so I cannot set a unique number of nops. I could
pass this information from arch_cmpxchg() down to __arch_cmpxchg() but
those macros are already complex enough so I'd rather not add another
parameter.

This suggestion unfortunately cannot be applied to
__arch_cmpxchg_masked(), nor __arch_xchg_masked().

So unless you and Conor really insist, I'll drop the idea!

Thanks,

Alex


>
> 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:                                                         \
>