Re: [PATCH v2 06/13] riscv: introduce riscv_has_extension_[un]likely()

Conor Dooley
2022-12-06T20:25:52Z

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Hey Jisheng,

Just a couple really minor bits here...

On Mon, Dec 05, 2022 at 01:46:25AM +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

s/that's to say, the hart/so a hart's/
s/any more//

> after booting, this chacteristic make it straightforward to use

"booting. This characteristic makes it"

> static branch to check one specific ISA extension is supported or not

"a static branch to check if a"

> 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@sholland.org/
> [2]https://lore.kernel.org/linux-arm-kernel/20220912162210.3626215-8-mark.rutland@arm.com/

Can you make these into Link: tags please (and drop the line between the
and the SoB)? So:

Link: https://lore.kernel.org/linux-riscv/20220922060958.44203-1-samuel@sholland.org/ [1]
Link: https://lore.kernel.org/linux-arm-kernel/20220912162210.3626215-8-mark.rutland@arm.com/ [2]
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> Reviewed-by: Andrew Jones <ajones@ventanamicro.com>

Changes themselves look grand, no comments there :)

Thanks!
Conor.

> ---
> 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 996884986fea..e2d3f6df7701 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_MAX,
> + "ext must be < RISCV_ISA_EXT_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_MAX,
> + "ext must be < RISCV_ISA_EXT_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
>

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