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

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Date:   Tue, 7 Dec 2021 10:21:26 +0800
From:   Yunfeng Ye <yeyunfeng@...wei.com>
To:     Catalin Marinas <catalin.marinas@....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 2021/12/7 0:23, Catalin Marinas wrote:
> 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.
> 
The commit 0c8ea531b774 ("arm64: mm: Allocate ASIDs in pairs") introduce the
asid2idx and idx2asid macro, but these macros is not really useful after the
commit f88f42f853a8 ("arm64: context: Free up kernel ASIDs if KPTI is not in use").

I think "(asid & ~ASID_MASK)" can be instead by a macro, it is the same code with
asid2idx(). Can it be renamed?  (eg, ctxid2asid)

Thanks.