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

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Message-ID: <20250526-baaca3f03adcac2b6488f040@orel>
Date: Mon, 26 May 2025 10:41:33 +0200
From: Andrew Jones <ajones@...tanamicro.com>
To: Clément Léger <cleger@...osinc.com>
Cc: Charlie Jenkins <charlie@...osinc.com>, 
	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>, 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 09:21:51PM +0200, Clément Léger wrote:
> 
> 
> 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 ?

Would it need an ifdef? Or can we just do

 if (!IS_ENABLED(CONFIG_RISCV_SCALAR_MISALIGNED))
    return false;

at the top of the function?

While the function wouldn't waste much time since it's not called much and
would return false on the first check done in the loop, since it's a
static function, adding the IS_ENABLED() check would likely allow the
compiler to completely remove it and all the branches depending on it.

Thanks,
drew

> 
> > 
> > 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
> >>
>