linux-kernel - Re: [PATCH v4 12/12] riscv: mm: Always use an ASID to flush mm contexts

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Message-ID: <e0ceb2e3-ae27-47aa-94de-f0d0060b2a77@sifive.com>
Date: Thu, 4 Jan 2024 09:50:48 -0600
From: Samuel Holland <samuel.holland@...ive.com>
To: Jisheng Zhang <jszhang@...nel.org>
Cc: Palmer Dabbelt <palmer@...belt.com>, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Alexandre Ghiti <alexghiti@...osinc.com>
Subject: Re: [PATCH v4 12/12] riscv: mm: Always use an ASID to flush mm
 contexts

On 2024-01-03 9:02 AM, Jisheng Zhang wrote:
> On Tue, Jan 02, 2024 at 02:00:49PM -0800, Samuel Holland wrote:
>> Even if multiple ASIDs are not supported, using the single-ASID variant
>> of the sfence.vma instruction preserves TLB entries for global (kernel)
>> pages. So it is always more efficient to use the single-ASID code path.
>>
>> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
>> ---
>>
>> Changes in v4:
>>  - There is now only one copy of __flush_tlb_range()
>>
>> Changes in v2:
>>  - Update both copies of __flush_tlb_range()
>>
>>  arch/riscv/include/asm/mmu_context.h | 2 --
>>  arch/riscv/mm/context.c              | 3 +--
>>  arch/riscv/mm/tlbflush.c             | 3 +--
>>  3 files changed, 2 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/riscv/include/asm/mmu_context.h b/arch/riscv/include/asm/mmu_context.h
>> index 7030837adc1a..b0659413a080 100644
>> --- a/arch/riscv/include/asm/mmu_context.h
>> +++ b/arch/riscv/include/asm/mmu_context.h
>> @@ -33,8 +33,6 @@ static inline int init_new_context(struct task_struct *tsk,
>>  	return 0;
>>  }
>>  
>> -DECLARE_STATIC_KEY_FALSE(use_asid_allocator);
>> -
>>  #include <asm-generic/mmu_context.h>
>>  
>>  #endif /* _ASM_RISCV_MMU_CONTEXT_H */
>> diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
>> index 3ca9b653df7d..20057085ab8a 100644
>> --- a/arch/riscv/mm/context.c
>> +++ b/arch/riscv/mm/context.c
>> @@ -18,8 +18,7 @@
>>  
>>  #ifdef CONFIG_MMU
>>  
>> -DEFINE_STATIC_KEY_FALSE(use_asid_allocator);
>> -
>> +static DEFINE_STATIC_KEY_FALSE(use_asid_allocator);
> 
> One of my optimization "riscv: tlb: avoid tlb flushing if fullmm == 1"
> will make use of use_asid_allocator, so could we remove this modification?

Yes, I can leave the global declaration alone for now.

>>  static unsigned long num_asids;
>>  
>>  static atomic_long_t current_version;
>> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
>> index 5ec621545c69..39d80f56d292 100644
>> --- a/arch/riscv/mm/tlbflush.c
>> +++ b/arch/riscv/mm/tlbflush.c
>> @@ -84,8 +84,7 @@ static void __flush_tlb_range(struct mm_struct *mm, unsigned long start,
>>  		if (cpumask_empty(cmask))
>>  			return;
>>  
>> -		if (static_branch_unlikely(&use_asid_allocator))
>> -			asid = cntx2asid(atomic_long_read(&mm->context.id));
>> +		asid = cntx2asid(atomic_long_read(&mm->context.id));
>>  	} else {
>>  		cmask = cpu_online_mask;
>>  	}
>> -- 
>> 2.42.0
>>
>>
>> _______________________________________________
>> linux-riscv mailing list
>> linux-riscv@...ts.infradead.org
>> http://lists.infradead.org/mailman/listinfo/linux-riscv

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