linux-kernel - Re: [PATCH] arm64: mm: Use asid2idx() and asid feature macro for cleanup

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Date:   Mon, 6 Dec 2021 16:23:20 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Yunfeng Ye <yeyunfeng@...wei.com>
Cc:     will@...nel.org, wangkefeng.wang@...wei.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        wuxu.wu@...wei.com, Hewenliang <hewenliang4@...wei.com>
Subject: Re: [PATCH] arm64: mm: Use asid2idx() and asid feature macro for
 cleanup

On Thu, Oct 28, 2021 at 08:27:23PM +0800, Yunfeng Ye wrote:
> Use asid2idx() and asid feature macro for cleanup.
> 
> No functional change.
> 
> Signed-off-by: Yunfeng Ye <yeyunfeng@...wei.com>
> ---
>  arch/arm64/mm/context.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/mm/context.c b/arch/arm64/mm/context.c
> index cd72576ae2b7..076f14a75bd5 100644
> --- a/arch/arm64/mm/context.c
> +++ b/arch/arm64/mm/context.c
> @@ -50,10 +50,10 @@ static u32 get_cpu_asid_bits(void)
>  		pr_warn("CPU%d: Unknown ASID size (%d); assuming 8-bit\n",
>  					smp_processor_id(),  fld);
>  		fallthrough;
> -	case 0:
> +	case ID_AA64MMFR0_ASID_8:
>  		asid = 8;
>  		break;
> -	case 2:
> +	case ID_AA64MMFR0_ASID_16:
>  		asid = 16;
>  	}

I think this change is fine.

> @@ -162,7 +162,7 @@ static u64 new_context(struct mm_struct *mm)
>  	u64 generation = atomic64_read(&asid_generation);
> 
>  	if (asid != 0) {
> -		u64 newasid = generation | (asid & ~ASID_MASK);
> +		u64 newasid = generation | asid2idx(asid);
> 
>  		/*
>  		 * If our current ASID was active during a rollover, we
> @@ -306,7 +306,7 @@ unsigned long arm64_mm_context_get(struct mm_struct *mm)
>  out_unlock:
>  	raw_spin_unlock_irqrestore(&cpu_asid_lock, flags);
> 
> -	asid &= ~ASID_MASK;
> +	asid = asid2idx(asid);

While functionally the code is the same, I don't think this was the
intention of asid2idx(). It's meant to provide an index into asid_map,
while the ASID_MASK lines isolate the asid number and add a new
generation to it.

-- 
Catalin