Re: [PATCH v3 05/11] riscv: Implement arch_cmpxchg128() using Zacas

Alexandre Ghiti
2024-07-18T07:48:55Z

Hi Drew,

On Wed, Jul 17, 2024 at 10:34 PM Andrew Jones <ajones@ventanamicro.com> wrote:
>
> On Wed, Jul 17, 2024 at 08:19:51AM GMT, Alexandre Ghiti wrote:
> > Now that Zacas is supported in the kernel, let's use the double word
> > atomic version of amocas to improve the SLUB allocator.
> >
> > Note that we have to select fixed registers, otherwise gcc fails to pick
> > even registers and then produces a reserved encoding which fails to
> > assemble.
>
> Oh, that's quite unfortunate... I guess we should try to get some new
> RISC-V inline assembly register constraints added to support register
> pairs.

I internally informed the compilers people, I'll check their progress.

>
> >
> > Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> > ---
> > arch/riscv/Kconfig | 1 +
> > arch/riscv/include/asm/cmpxchg.h | 39 ++++++++++++++++++++++++++++++++
> > 2 files changed, 40 insertions(+)
> >
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index d3b0f92f92da..0bbaec0444d0 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -104,6 +104,7 @@ config RISCV
> > select GENERIC_VDSO_TIME_NS if HAVE_GENERIC_VDSO
> > select HARDIRQS_SW_RESEND
> > select HAS_IOPORT if MMU
> > + select HAVE_ALIGNED_STRUCT_PAGE
> > select HAVE_ARCH_AUDITSYSCALL
> > select HAVE_ARCH_HUGE_VMALLOC if HAVE_ARCH_HUGE_VMAP
> > select HAVE_ARCH_HUGE_VMAP if MMU && 64BIT
> > diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> > index 97b24da38897..608d98522557 100644
> > --- a/arch/riscv/include/asm/cmpxchg.h
> > +++ b/arch/riscv/include/asm/cmpxchg.h
> > @@ -289,4 +289,43 @@ end:; \
> > arch_cmpxchg_release((ptr), (o), (n)); \
> > })
> >
> > +#ifdef CONFIG_RISCV_ISA_ZACAS
>
> This is also 64-bit only, so needs a CONFIG_64BIT check too.

Yep, thanks

>
> > +
> > +#define system_has_cmpxchg128() \
> > + riscv_has_extension_unlikely(RISCV_ISA_EXT_ZACAS)
>
> nit: let's let this stick out since we have 100 chars

Ok

>
> > +
> > +union __u128_halves {
> > + u128 full;
> > + struct {
> > + u64 low, high;
>
> Should we consider big endian too?

Should we care about big endian? We don't deal with big endian
anywhere in our kernel right now.

>
> > + };
> > +};
> > +
> > +#define __arch_cmpxchg128(p, o, n, cas_sfx) \
> > +({ \
> > + __typeof__(*(p)) __o = (o); \
> > + union __u128_halves __hn = { .full = (n) }; \
> > + union __u128_halves __ho = { .full = (__o) }; \
> > + register unsigned long x6 asm ("x6") = __hn.low; \
> > + register unsigned long x7 asm ("x7") = __hn.high; \
> > + register unsigned long x28 asm ("x28") = __ho.low; \
> > + register unsigned long x29 asm ("x29") = __ho.high; \
>
> Can we use t1,t2,t3,t4 rather than the x names?

We can, I did not because it was a bit misleading in the sense that
amocas expects an *even* register and using the tX registers, we'll
pass an *odd* register (which actually is even but still).

Anyway, I'll change that, I don't like the xX notation.

Thanks for the review,

Alex

>
> > + \
> > + __asm__ __volatile__ ( \
> > + " amocas.q" cas_sfx " %0, %z3, %2" \
> > + : "+&r" (x28), "+&r" (x29), "+A" (*(p)) \
> > + : "rJ" (x6), "rJ" (x7) \
> > + : "memory"); \
> > + \
> > + ((u128)x29 << 64) | x28; \
> > +})
> > +
> > +#define arch_cmpxchg128(ptr, o, n) \
> > + __arch_cmpxchg128((ptr), (o), (n), ".aqrl")
> > +
> > +#define arch_cmpxchg128_local(ptr, o, n) \
> > + __arch_cmpxchg128((ptr), (o), (n), "")
> > +
> > +#endif /* CONFIG_RISCV_ISA_ZACAS */
> > +
> > #endif /* _ASM_RISCV_CMPXCHG_H */
> > --
> > 2.39.2
>
> Thanks,
> drew