Re: [PATCH v3 12/15] arm64/mm: Split __flush_tlb_range() to elide trailing DSB

[Will Deacon]

On Mon, Dec 04, 2023 at 10:54:37AM +0000, Ryan Roberts wrote:
> Split __flush_tlb_range() into __flush_tlb_range_nosync() +
> __flush_tlb_range(), in the same way as the existing flush_tlb_page()
> arrangement. This allows calling __flush_tlb_range_nosync() to elide the
> trailing DSB. Forthcoming "contpte" code will take advantage of this
> when clearing the young bit from a contiguous range of ptes.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---
>  arch/arm64/include/asm/tlbflush.h | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
> index bb2c2833a987..925ef3bdf9ed 100644
> --- a/arch/arm64/include/asm/tlbflush.h
> +++ b/arch/arm64/include/asm/tlbflush.h
> @@ -399,7 +399,7 @@ do {									\
>  #define __flush_s2_tlb_range_op(op, start, pages, stride, tlb_level) \
>  	__flush_tlb_range_op(op, start, pages, stride, 0, tlb_level, false)
>  
> -static inline void __flush_tlb_range(struct vm_area_struct *vma,
> +static inline void __flush_tlb_range_nosync(struct vm_area_struct *vma,
>  				     unsigned long start, unsigned long end,
>  				     unsigned long stride, bool last_level,
>  				     int tlb_level)
> @@ -431,10 +431,19 @@ static inline void __flush_tlb_range(struct vm_area_struct *vma,
>  	else
>  		__flush_tlb_range_op(vae1is, start, pages, stride, asid, tlb_level, true);
>  
> -	dsb(ish);
>  	mmu_notifier_arch_invalidate_secondary_tlbs(vma->vm_mm, start, end);
>  }
>  
> +static inline void __flush_tlb_range(struct vm_area_struct *vma,
> +				     unsigned long start, unsigned long end,
> +				     unsigned long stride, bool last_level,
> +				     int tlb_level)
> +{
> +	__flush_tlb_range_nosync(vma, start, end, stride,
> +				 last_level, tlb_level);
> +	dsb(ish);
> +}

Hmm, are you sure it's safe to defer the DSB until after the secondary TLB
invalidation? It will have a subtle effect on e.g. an SMMU participating
in broadcast TLB maintenance, because now the ATC will be invalidated
before completion of the TLB invalidation and it's not obviously safe to me.

Will