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SubjectRe: [PATCH] arch/riscv: Add Zihintpause extension support
On Fri, May 13, 2022 at 12:09 AM Atish Patra <atishp@atishpatra.org> wrote:
>
> On Thu, May 12, 2022 at 6:06 PM Samuel Holland <samuel@sholland.org> wrote:
> >
> > On 5/11/22 10:30 PM, Dao Lu wrote:
> > > This patch:
> > > 1. Build with _zihintpause if the toolchain has support for it
> > > 2. Detects if the platform supports the extension
> >
> > This instruction is a hint, meaning it is a harmless no-op if the extension is
> > unsupported by the CPU. So we can use it as long as the compiler supports it.
> > There is no need to probe for it at runtime.
> >
>
> Is it guaranteed that the hardware won't throw any error if it sees a
> fence instruction with
> (pred=W, succ=0, fm=0, rd=x0, and rs1=x0.) ? I couldn't find anything
> specific related to this in the spec.
>

Never mind. I found this

"The FENCE encoding currently has nine non-trivial combinations of the
four bits PR, PW, SR,
and SW, plus one extra encoding FENCE.TSO which facilitates mapping of
“acquire+release” or
RVTSO semantics. The remaining seven combinations have empty
predecessor and/or successor
sets and hence are no-ops."

However, we still need the extension availability check to preserve
the older platform's behavior.
Currently, the stall in cpu_relax is caused by the div. Without the
extension probe, it will just execute
"nop" which was not the earlier behavior.

> I think using the static key mechanism provides backward compatibility
> without any runtime impact.
>
> > Regards,
> > Samuel
> >
> > > 3. Use PAUSE for cpu_relax if both toolchain and the platform support it
> > >
> > > Signed-off-by: Dao Lu <daolu@rivosinc.com>
> > > ---
> > > arch/riscv/Makefile | 4 ++++
> > > arch/riscv/include/asm/hwcap.h | 1 +
> > > arch/riscv/include/asm/vdso/processor.h | 19 ++++++++++++++++---
> > > arch/riscv/kernel/cpu.c | 1 +
> > > arch/riscv/kernel/cpufeature.c | 7 +++++++
> > > 5 files changed, 29 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> > > index 7d81102cffd4..900a8fda1a2d 100644
> > > --- a/arch/riscv/Makefile
> > > +++ b/arch/riscv/Makefile
> > > @@ -56,6 +56,10 @@ riscv-march-$(CONFIG_RISCV_ISA_C) := $(riscv-march-y)c
> > > toolchain-need-zicsr-zifencei := $(call cc-option-yn, -march=$(riscv-march-y)_zicsr_zifencei)
> > > riscv-march-$(toolchain-need-zicsr-zifencei) := $(riscv-march-y)_zicsr_zifencei
> > >
> > > +# Check if the toolchain supports Zihintpause extension
> > > +toolchain-supports-zihintpause := $(call cc-option-yn, -march=$(riscv-march-y)_zihintpause)
> > > +riscv-march-$(toolchain-supports-zihintpause) := $(riscv-march-y)_zihintpause
> > > +
> > > KBUILD_CFLAGS += -march=$(subst fd,,$(riscv-march-y))
> > > KBUILD_AFLAGS += -march=$(riscv-march-y)
> > >
> > > diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
> > > index 0734e42f74f2..caa9ee5459b4 100644
> > > --- a/arch/riscv/include/asm/hwcap.h
> > > +++ b/arch/riscv/include/asm/hwcap.h
> > > @@ -52,6 +52,7 @@ extern unsigned long elf_hwcap;
> > > */
> > > enum riscv_isa_ext_id {
> > > RISCV_ISA_EXT_SSCOFPMF = RISCV_ISA_EXT_BASE,
> > > + RISCV_ISA_EXT_ZIHINTPAUSE,
> > > RISCV_ISA_EXT_ID_MAX = RISCV_ISA_EXT_MAX,
> > > };
> > >
> > > diff --git a/arch/riscv/include/asm/vdso/processor.h b/arch/riscv/include/asm/vdso/processor.h
> > > index 134388cbaaa1..106b35ba8cac 100644
> > > --- a/arch/riscv/include/asm/vdso/processor.h
> > > +++ b/arch/riscv/include/asm/vdso/processor.h
> > > @@ -4,15 +4,28 @@
> > >
> > > #ifndef __ASSEMBLY__
> > >
> > > +#include <linux/jump_label.h>
> > > #include <asm/barrier.h>
> > > +#include <asm/hwcap.h>
> > >
> > > +extern struct static_key_false riscv_pause_available;
> > > static inline void cpu_relax(void)
> > > {
> > > + if (!static_branch_likely(&riscv_pause_available)) {
> > > #ifdef __riscv_muldiv
> > > - int dummy;
> > > - /* In lieu of a halt instruction, induce a long-latency stall. */
> > > - __asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
> > > + int dummy;
> > > + /* In lieu of a halt instruction, induce a long-latency stall. */
> > > + __asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
> > > #endif
> > > + } else {
> > > +#ifdef __riscv_zihintpause
> > > + /*
> > > + * Reduce instruction retirement.
> > > + * This assumes the PC changes.
> > > + */
> > > + __asm__ __volatile__ ("pause");
> > > +#endif
> > > + }
> > > barrier();
> > > }
> > >
> > > diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
> > > index ccb617791e56..89e563e9c4cc 100644
> > > --- a/arch/riscv/kernel/cpu.c
> > > +++ b/arch/riscv/kernel/cpu.c
> > > @@ -88,6 +88,7 @@ int riscv_of_parent_hartid(struct device_node *node)
> > > */
> > > static struct riscv_isa_ext_data isa_ext_arr[] = {
> > > __RISCV_ISA_EXT_DATA(sscofpmf, RISCV_ISA_EXT_SSCOFPMF),
> > > + __RISCV_ISA_EXT_DATA(zihintpause, RISCV_ISA_EXT_ZIHINTPAUSE),
> > > __RISCV_ISA_EXT_DATA("", RISCV_ISA_EXT_MAX),
> > > };
> > >
> > > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > > index 1b2d42d7f589..327c19507dbb 100644
> > > --- a/arch/riscv/kernel/cpufeature.c
> > > +++ b/arch/riscv/kernel/cpufeature.c
> > > @@ -24,6 +24,8 @@ static DECLARE_BITMAP(riscv_isa, RISCV_ISA_EXT_MAX) __read_mostly;
> > > #ifdef CONFIG_FPU
> > > __ro_after_init DEFINE_STATIC_KEY_FALSE(cpu_hwcap_fpu);
> > > #endif
> > > +DEFINE_STATIC_KEY_FALSE(riscv_pause_available);
> > > +EXPORT_SYMBOL_GPL(riscv_pause_available);
> > >
> > > /**
> > > * riscv_isa_extension_base() - Get base extension word
> > > @@ -192,6 +194,7 @@ void __init riscv_fill_hwcap(void)
> > > set_bit(*ext - 'a', this_isa);
> > > } else {
> > > SET_ISA_EXT_MAP("sscofpmf", RISCV_ISA_EXT_SSCOFPMF);
> > > + SET_ISA_EXT_MAP("zihintpause", RISCV_ISA_EXT_ZIHINTPAUSE);
> > > }
> > > #undef SET_ISA_EXT_MAP
> > > }
> > > @@ -213,6 +216,10 @@ void __init riscv_fill_hwcap(void)
> > >
> > > }
> > >
> > > + if (__riscv_isa_extension_available(riscv_isa, RISCV_ISA_EXT_ZIHINTPAUSE)) {
> > > + static_branch_enable(&riscv_pause_available);
> > > + }
> > > +
> > > /* We don't support systems with F but without D, so mask those out
> > > * here. */
> > > if ((elf_hwcap & COMPAT_HWCAP_ISA_F) && !(elf_hwcap & COMPAT_HWCAP_ISA_D)) {
> > >
> >
> >
> > _______________________________________________
> > linux-riscv mailing list
> > linux-riscv@lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-riscv
>
>
>
> --
> Regards,
> Atish



--
Regards,
Atish

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