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

[Jisheng Zhang <jszhang@kernel.org>]

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@sifive.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?

>  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
> 
> 
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