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

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Message-ID: <b2afb9c7-a3d2-4bf6-bfaa-d804358ccd88@rivosinc.com>
Date: Fri, 23 May 2025 21:21:51 +0200
From: Clément Léger <cleger@...osinc.com>
To: Charlie Jenkins <charlie@...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 23/05/2025 20:30, Charlie Jenkins wrote:
> 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.

Yeah I though of that as well but the amount of call to this function is
probably well below 10 times so I guess it does not really matters in
that case to justify yet another ifdef ?

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

Thanks,

Clément

> 
>> +
>>  #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
>>