Re: [PATCH 5/8] riscv: introduce riscv_has_extension_[un]likely()

[Andrew Jones]

On Thu, Oct 06, 2022 at 03:08:15PM +0800, Jisheng Zhang wrote:
> Generally, riscv ISA extensions are fixed for any specific hardware
> platform, that's to say, the hart features won't change any more
> after booting, this chacteristic make it straightforward to use
> static branch to check one specific ISA extension is supported or not
> to optimize performance.
> 
> However, some ISA extensions such as SVPBMT and ZICBOM are handled
> via. the alternative sequences.
> 
> Basically, for ease of maintenance, we prefer to use static branches
> in C code, but recently, Samuel found that the static branch usage in
> cpu_relax() breaks building with CONFIG_CC_OPTIMIZE_FOR_SIZE[1]. As
> Samuel pointed out, "Having a static branch in cpu_relax() is
> problematic because that function is widely inlined, including in some
> quite complex functions like in the VDSO. A quick measurement shows
> this static branch is responsible by itself for around 40% of the jump
> table."
> 
> Samuel's findings pointed out one of a few downsides of static branches
> usage in C code to handle ISA extensions detected at boot time:
> static branch's metadata in the __jump_table section, which is not
> discarded after ISA extensions are finalized, wastes some space.
> 
> I want to try to solve the issue for all possible dynamic handling of
> ISA extensions at boot time. Inspired by Mark[2], this patch introduces
> riscv_has_extension_*() helpers, which work like static branches but
> are patched using alternatives, thus the metadata can be freed after
> patching.
> 
> [1]https://lore.kernel.org/linux-riscv/20220922060958.44203-1-samuel@xxxxxxxxxxxx/
> [2]https://lore.kernel.org/linux-arm-kernel/20220912162210.3626215-8-mark.rutland@xxxxxxx/
> 
> Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
> ---
>  arch/riscv/include/asm/hwcap.h | 37 ++++++++++++++++++++++++++++++++++
>  1 file changed, 37 insertions(+)
> 
> diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
> index 6cf445653911..54b88ee6cae1 100644
> --- a/arch/riscv/include/asm/hwcap.h
> +++ b/arch/riscv/include/asm/hwcap.h
> @@ -8,6 +8,7 @@
>  #ifndef _ASM_RISCV_HWCAP_H
>  #define _ASM_RISCV_HWCAP_H
>  
> +#include <asm/alternative-macros.h>
>  #include <asm/errno.h>
>  #include <linux/bits.h>
>  #include <uapi/asm/hwcap.h>
> @@ -96,6 +97,42 @@ static __always_inline int riscv_isa_ext2key(int num)
>  	}
>  }
>  
> +static __always_inline bool
> +riscv_has_extension_likely(const unsigned long ext)
> +{
> +	compiletime_assert(ext < RISCV_ISA_EXT_ID_MAX,
> +			   "ext must be < RISCV_ISA_EXT_ID_MAX");
> +
> +	asm_volatile_goto(
> +	ALTERNATIVE("j	%l[l_no]", "nop", 0, %[ext], 1)
> +	:
> +	: [ext] "i" (ext)
> +	:
> +	: l_no);
> +
> +	return true;
> +l_no:
> +	return false;
> +}
> +
> +static __always_inline bool
> +riscv_has_extension_unlikely(const unsigned long ext)
> +{
> +	compiletime_assert(ext < RISCV_ISA_EXT_ID_MAX,
> +			   "ext must be < RISCV_ISA_EXT_ID_MAX");
> +
> +	asm_volatile_goto(
> +	ALTERNATIVE("nop", "j	%l[l_yes]", 0, %[ext], 1)
> +	:
> +	: [ext] "i" (ext)
> +	:
> +	: l_yes);
> +
> +	return false;
> +l_yes:
> +	return true;
> +}
> +
>  unsigned long riscv_isa_extension_base(const unsigned long *isa_bitmap);
>  
>  #define riscv_isa_extension_mask(ext) BIT_MASK(RISCV_ISA_EXT_##ext)
> -- 
> 2.37.2
>

Cheers to Mark for this nifty idea and to you for bringing it to riscv.

Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>