Re: [PATCH v4 6/8] RISC-V: Use Zicboz in clear_page when available

[Conor Dooley <conor@kernel.org>]

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On Thu, Feb 09, 2023 at 04:26:26PM +0100, Andrew Jones wrote:
> Using memset() to zero a 4K page takes 563 total instructions, where
> 20 are branches. clear_page(), with Zicboz and a 64 byte block size,
> takes 169 total instructions, where 4 are branches and 33 are nops.
> Even though the block size is a variable, thanks to alternatives, we
> can still implement a Duff device without having to do any preliminary
> calculations. This is achieved by taking advantage of 'vendor_id'
> being used as application-specific data for alternatives, enabling us
> to stop patching / unrolling when 4K bytes have been zeroed (we would
> loop and continue after 4K if the page size would be larger)
> 
> For 4K pages, unrolling 16 times allows block sizes of 64 and 128 to
> only loop a few times and larger block sizes to not loop at all. Since
> cbo.zero doesn't take an offset, we also need an 'add' after each
> instruction, making the loop body 112 to 160 bytes. Hopefully this
> is small enough to not cause icache misses.
> 
> Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
> Acked-by: Conor Dooley <conor.dooley@microchip.com>

> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 74736b4f0624..42246bbfa532 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -280,6 +280,17 @@ void __init riscv_fill_hwcap(void)
>  #ifdef CONFIG_RISCV_ALTERNATIVE
>  static bool riscv_cpufeature_application_check(u32 feature, u16 data)
>  {
> +	switch (feature) {
> +	case RISCV_ISA_EXT_ZICBOZ:
> +		/*
> +		 * Zicboz alternative applications provide the maximum

I like the comment, rather than this being some wizardry.
I find the word "applications" to be a little unclear, perhaps, iff this
series needs a respin, this would work better as "Users of the Zicboz
alternative provide..." (or s/Users/Callers)?

> +		 * supported block size order, or zero when it doesn't
> +		 * matter. If the current block size exceeds the maximum,
> +		 * then the alternative cannot be applied.
> +		 */
> +		return data == 0 || riscv_cboz_block_size <= (1U << data);
> +	}
> +
>  	return data == 0;
>  }

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