On Wed, Jun 25, 2025 at 04:21:01PM +0200, Aleksa Paunovic via B4 Relay wrote:
> From: Aleksa Paunovic <aleksa.paunovic@xxxxxxxxxxxxx>
>
> Use the hwprobe syscall to decide which PAUSE instruction to execute in
> userspace code.
>
> Signed-off-by: Aleksa Paunovic <aleksa.paunovic@xxxxxxxxxxxxx>
> ---
> tools/arch/riscv/include/asm/vdso/processor.h | 27 +++++++++++++++++----------
> 1 file changed, 17 insertions(+), 10 deletions(-)
>
> diff --git a/tools/arch/riscv/include/asm/vdso/processor.h b/tools/arch/riscv/include/asm/vdso/processor.h
> index 662aca03984817f9c69186658b19e9dad9e4771c..027219a486b7b93814888190f8224af29498707c 100644
> --- a/tools/arch/riscv/include/asm/vdso/processor.h
> +++ b/tools/arch/riscv/include/asm/vdso/processor.h
> @@ -4,26 +4,33 @@
>
> #ifndef __ASSEMBLY__
>
> +#include <asm/hwprobe.h>
> +#include <sys/hwprobe.h>
> +#include <asm/vendor/mips.h>
> #include <asm-generic/barrier.h>
>
> static inline void cpu_relax(void)
> {
> + struct riscv_hwprobe pair;
> + bool has_mipspause;
> #ifdef __riscv_muldiv
> int dummy;
> /* In lieu of a halt instruction, induce a long-latency stall. */
> __asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
> #endif
>
> -#ifdef CONFIG_TOOLCHAIN_HAS_ZIHINTPAUSE
> - /*
> - * Reduce instruction retirement.
> - * This assumes the PC changes.
> - */
> - __asm__ __volatile__ ("pause");
> -#else
> - /* Encoding of the pause instruction */
> - __asm__ __volatile__ (".4byte 0x100000F");
> -#endif
> + pair.key = RISCV_HWPROBE_KEY_VENDOR_EXT_MIPS_0;
> + __riscv_hwprobe(&pair, 1, 0, NULL, 0);
> + has_mipspause = pair.value & RISCV_HWPROBE_VENDOR_EXT_XMIPSEXECTL;
> +
> + if (has_mipspause) {
> + /* Encoding of the mips pause instruction */
> + __asm__ __volatile__(".4byte 0x00501013");
> + } else {
> + /* Encoding of the pause instruction */
> + __asm__ __volatile__(".4byte 0x100000F");
> + }
> +
cpu_relax() is used in places where we cannot afford the overhead nor call
arbitrary functions which may take locks, etc. We've even had trouble
using a static key here in the past since this is inlined and it bloated
the size too much. You'll need to use ALTERNATIVE().
Thanks,
drew
> barrier();
> }
>
>
> --
> 2.34.1
>
>
>
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