linux-kernel - Re: [RFC PATCH 02/11] arm64: tlb: Add DSB ISHST prior to TLBI in __flush_tlb_[kernel

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Date:   Tue, 28 Aug 2018 14:03:27 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, benh@....ibm.com,
        torvalds@...ux-foundation.org, npiggin@...il.com,
        catalin.marinas@....com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH 02/11] arm64: tlb: Add DSB ISHST prior to TLBI in
 __flush_tlb_[kernel_]pgtable()

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@....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it's desperately out of date, so lemme rewrite that and justify the barriers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