linux-kernel - Re: [PATCH v8 09/14] riscv: misaligned: move emulated access uniformity check in a function

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Message-ID: <aDC-0qe5STR7ow4m@ghost>
Date: Fri, 23 May 2025 11:30:42 -0700
From: Charlie Jenkins <charlie@...osinc.com>
To: Clément Léger <cleger@...osinc.com>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Anup Patel <anup@...infault.org>,
	Atish Patra <atishp@...shpatra.org>, Shuah Khan <shuah@...nel.org>,
	Jonathan Corbet <corbet@....net>, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org,
	linux-kselftest@...r.kernel.org,
	Samuel Holland <samuel.holland@...ive.com>,
	Andrew Jones <ajones@...tanamicro.com>,
	Deepak Gupta <debug@...osinc.com>
Subject: Re: [PATCH v8 09/14] riscv: misaligned: move emulated access
 uniformity check in a function

On Fri, May 23, 2025 at 12:19:26PM +0200, Clément Léger wrote:
> Split the code that check for the uniformity of misaligned accesses
> performance on all cpus from check_unaligned_access_emulated_all_cpus()
> to its own function which will be used for delegation check. No
> functional changes intended.
> 
> Signed-off-by: Clément Léger <cleger@...osinc.com>
> Reviewed-by: Andrew Jones <ajones@...tanamicro.com>
> ---
>  arch/riscv/kernel/traps_misaligned.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
> index f1b2af515592..7ecaa8103fe7 100644
> --- a/arch/riscv/kernel/traps_misaligned.c
> +++ b/arch/riscv/kernel/traps_misaligned.c
> @@ -645,6 +645,18 @@ bool __init check_vector_unaligned_access_emulated_all_cpus(void)
>  }
>  #endif
>  
> +static bool all_cpus_unaligned_scalar_access_emulated(void)
> +{
> +	int cpu;
> +
> +	for_each_online_cpu(cpu)
> +		if (per_cpu(misaligned_access_speed, cpu) !=
> +		    RISCV_HWPROBE_MISALIGNED_SCALAR_EMULATED)
> +			return false;
> +
> +	return true;
> +}

This ends up wasting time when !CONFIG_RISCV_SCALAR_MISALIGNED since it
will always return false in that case. Maybe there is a way to simplify
the ifdefs and still have performant code, but I don't think this is a
big enough problem to prevent this patch from merging.

Reviewed-by: Charlie Jenkins <charlie@...osinc.com>
Tested-by: Charlie Jenkins <charlie@...osinc.com>

> +
>  #ifdef CONFIG_RISCV_SCALAR_MISALIGNED
>  
>  static bool unaligned_ctl __read_mostly;
> @@ -683,8 +695,6 @@ static int cpu_online_check_unaligned_access_emulated(unsigned int cpu)
>  
>  bool __init check_unaligned_access_emulated_all_cpus(void)
>  {
> -	int cpu;
> -
>  	/*
>  	 * We can only support PR_UNALIGN controls if all CPUs have misaligned
>  	 * accesses emulated since tasks requesting such control can run on any
> @@ -692,10 +702,8 @@ bool __init check_unaligned_access_emulated_all_cpus(void)
>  	 */
>  	on_each_cpu(check_unaligned_access_emulated, NULL, 1);
>  
> -	for_each_online_cpu(cpu)
> -		if (per_cpu(misaligned_access_speed, cpu)
> -		    != RISCV_HWPROBE_MISALIGNED_SCALAR_EMULATED)
> -			return false;
> +	if (!all_cpus_unaligned_scalar_access_emulated())
> +		return false;
>  
>  	unaligned_ctl = true;
>  	return true;
> -- 
> 2.49.0
>