Subject: [PATCH 1/4] ARM: tlb: don't perform inner-shareable invalidation for local TLB ops

From: [email-masked] (Catalin Marinas)
Date: Wed, 27 Mar 2013 13:40:29 +0000

On Wed, Mar 27, 2013 at 12:56:39PM +0000, Will Deacon wrote:
> On Wed, Mar 27, 2013 at 12:30:55PM +0000, Catalin Marinas wrote:
> > On Wed, Mar 27, 2013 at 12:07:37PM +0000, Will Deacon wrote:
> > > On Wed, Mar 27, 2013 at 10:34:30AM +0000, Catalin Marinas wrote:
> > > > On Mon, Mar 25, 2013 at 06:19:38PM +0000, Will Deacon wrote:
> > > > > @@ -352,22 +369,33 @@ static inline void local_flush_tlb_mm(struct mm_struct *mm)
> > > > >  		dsb();
> > > > >  
> > > > >  	if (possible_tlb_flags & (TLB_V3_FULL|TLB_V4_U_FULL|TLB_V4_D_FULL|TLB_V4_I_FULL)) {
> > > > > -		if (cpumask_test_cpu(get_cpu(), mm_cpumask(mm))) {
> > > > > +		if (!cpumask_test_cpu(smp_processor_id(), mm_cpumask(mm))) {
> > > > >  			tlb_op(TLB_V3_FULL, "c6, c0, 0", zero);
> > > > >  			tlb_op(TLB_V4_U_FULL, "c8, c7, 0", zero);
> > > > >  			tlb_op(TLB_V4_D_FULL, "c8, c6, 0", zero);
> > > > >  			tlb_op(TLB_V4_I_FULL, "c8, c5, 0", zero);
> > > > >  		}
> > > > > -		put_cpu();
> > > > 
> > > > Why is this change needed? You only flush the local TLB if the mm never
> > > > wasn't active on this processor?
> > > 
> > > Ouch, that's a cock-up, sorry. I'll remove the '!'.
> > 
> > Do we also need to disable preemtion?
> 
> I don't think so, that should be taken care of by the caller if they are
> issuing the local_ operation (otherwise it's racy anyway).

OK.

> > > > >  #ifdef CONFIG_ARM_ERRATA_720789
> > > > >  	tlb_op(TLB_V7_UIS_PAGE, "c8, c3, 3", uaddr & PAGE_MASK);
> > > > >  #else
> > > > > @@ -428,6 +471,22 @@ static inline void local_flush_tlb_kernel_page(unsigned long kaddr)
> > > > >  	tlb_op(TLB_V6_U_PAGE, "c8, c7, 1", kaddr);
> > > > >  	tlb_op(TLB_V6_D_PAGE, "c8, c6, 1", kaddr);
> > > > >  	tlb_op(TLB_V6_I_PAGE, "c8, c5, 1", kaddr);
> > > > > +
> > > > > +	if (tlb_flag(TLB_BARRIER)) {
> > > > > +		dsb();
> > > > > +		isb();
> > > > > +	}
> > > > > +}
> > > > 
> > > > I have some worries with this function. It is used by set_top_pte() and
> > > > it really doesn't look like it has local-only semantics. For example,
> > > > you use it do flush the I-cache aliases and this must target all the
> > > > CPUs because of speculative prefetches, which means that set_top_pte()
> > > > must set the new alias on all the CPUs.
> > > 
> > > This looks like a bug in set_top_pte when it's called for cache-flushing.
> > > However, the only core this would affect is 11MPCore, which uses the
> > > ipi-based flushing anyway, so I think we're ok.
> > 
> > I don't think its 11MPCore only, set_top_pte() is called by
> > flush_icache_alias() from flush_ptrace_access() even on ARMv7.
> 
> Damn, yes, I missed those. Perhaps we should add set_top_pte_atomic, which
> just does the local flush, and then promote the current flush to be IS?

Where would we use the set_top_pte_atomic() on ARMv7?

> > > > Highmem mappings need to be revisited as well.
> > > 
> > > I think they're ok. Everything is either done in atomic context or under a
> > > raw spinlock, so the mappings aren't expected to be used by other CPUs.
> > 
> > It's not whether they are used explicitly but whether a speculative TLB
> > load can bring them in on a different CPU. I don't immediately see a
> > problem with non-aliasing caches but needs some more thinking.
> 
> But why do we care about the speculation? If the core doing the speculating
> is always going to write a new pte before dereferencing anything mapped
> there, then it will invalidate its own TLB then.

It's about speculation on another CPU.

Let's say CPU0 does several kmap_atomic() calls which in turn call
set_top_pte(). The same page tables are visible to CPU1 which
speculatively loads some top pte (not the latest). At this point we have
a VA pointing to different PAs on CPU0 and CPU1. CPU1 would not access
this VA, so not a problem here, but whether this matters for
inner-shareable cache maintenance (dma_cache_maint_page), I can't tell
yet (internal thread with the architecture guys).

-- 
Catalin