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

Will Deacon
2018-08-28T13:03:18Z

On Fri, Aug 24, 2018 at 07:56:09PM +0200, Peter Zijlstra wrote:
> 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.

Yeah, not a bad idea. We already have a big block comment in this file but
it's desperately out of date, so lemme rewrite that and justify the barriers
at the same time.

Will