Re: [RFC PATCH 02/11] arm64: tlb: Add DSB ISHST prior to TLBI in __flush_tlb_[kernel_]pgtable()

Peter Zijlstra
2018-08-24T17:56:24Z

On Fri, Aug 24, 2018 at 04:52:37PM +0100, Will Deacon wrote:
> __flush_tlb_[kernel_]pgtable() rely on set_pXd() having a DSB after
> writing the new table entry and therefore avoid the barrier prior to the
> TLBI instruction.
>
> In preparation for delaying our walk-cache invalidation on the unmap()
> path, move the DSB into the TLB invalidation routines.
>
> Signed-off-by: Will Deacon <will.deacon@arm.com>
> ---
> arch/arm64/include/asm/tlbflush.h | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
> index 7e2a35424ca4..e257f8655b84 100644
> --- a/arch/arm64/include/asm/tlbflush.h
> +++ b/arch/arm64/include/asm/tlbflush.h
> @@ -213,6 +213,7 @@ static inline void __flush_tlb_pgtable(struct mm_struct *mm,
> {
> unsigned long addr = __TLBI_VADDR(uaddr, ASID(mm));
>
> + dsb(ishst);
> __tlbi(vae1is, addr);
> __tlbi_user(vae1is, addr);
> dsb(ish);
> @@ -222,6 +223,7 @@ static inline void __flush_tlb_kernel_pgtable(unsigned long kaddr)
> {
> unsigned long addr = __TLBI_VADDR(kaddr, 0);
>
> + dsb(ishst);
> __tlbi(vaae1is, addr);
> dsb(ish);
> }

I would suggest these barrier -- like any other barriers, carry a
comment that explain the required ordering.

I think this here reads like:

STORE: unhook page

DSB-ishst: wait for all stores to complete
TLBI: invalidate broadcast
DSB-ish: wait for TLBI to complete

And the 'newly' placed DSB-ishst ensures the page is observed unlinked
before we issue the invalidate.