Re: [RFC PATCH] arm64: Elide dsb in kernel TLB invalidations

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, May 22, 2025 at 05:14:14PM +0530, Dev Jain wrote:
> dsb(ishst) is used to ensure that prior pagetable updates are completed.
> But, set_pmd/set_pud etc already issue a dsb-isb sequence for the exact
> same purpose. Therefore, we can elide the dsb in kernel tlb invalidation.
> 
> There were no issues observed while running mm selftests, including
> test_vmalloc.sh selftest to stress the vmalloc subsystem.
> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>  arch/arm64/include/asm/tlbflush.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
> index eba1a98657f1..9b4adf1ee45e 100644
> --- a/arch/arm64/include/asm/tlbflush.h
> +++ b/arch/arm64/include/asm/tlbflush.h
> @@ -508,7 +508,7 @@ static inline void flush_tlb_kernel_range(unsigned long start, unsigned long end
>  		return;
>  	}
>  
> -	dsb(ishst);
> +	/* dsb(ishst) not needed as callers (set_pxd) have that */
>  	__flush_tlb_range_op(vaale1is, start, pages, stride, 0,
>  			     TLBI_TTL_UNKNOWN, false, lpa2_is_enabled());
>  	dsb(ish);
> @@ -523,7 +523,7 @@ static inline void __flush_tlb_kernel_pgtable(unsigned long kaddr)
>  {
>  	unsigned long addr = __TLBI_VADDR(kaddr, 0);
>  
> -	dsb(ishst);
> +	/* dsb(ishst) not needed as callers (set_pxd) have that */
>  	__tlbi(vaae1is, addr);
>  	dsb(ish);
>  	isb();

What about __set_pte()? We only issue (or rather queue) a barrier if we
set a valid kernel pte. I recall we added them for the case where a TLBI
won't happen, see commit 7f0b1bf04511 ("arm64: Fix barriers used for
page table modifications"). When we clear a PTE, we rely on the TLB
maintenance to issue barriers.

Maybe something that can be optimised following Ryan's reworking but I
don't think it is safe to remove them now, given that
__set_pte_complete() skips the barriers for invalid ptes. Possibly the
__flush_tlb_kernel_pgtable() is alright but not the
flush_tlb_kernel_range() one.

-- 
Catalin