[PATCH] ARM: Add TLB flushing for both entries in a PMD

[Changhwan Youn]

So practically Cortex A9 doesn't need tlb_add_flush() in pte_free_tlb().
I was a little confused because of the address of tlb_add_flush().

I really appreciate your answering.

On Friday, November 25, 2011 7:18 PM, Catalin Marinas wrote
> On Fri, Nov 25, 2011 at 08:50:48AM +0000, Changhwan Youn wrote:
> > On Wednesday, November 23, 2011 7:46 PM, Catalin Marinas wrote:
> > > To: Changhwan Youn
> > > Cc: linux-arm-kernel at lists.infradead.org
> > > Subject: Re: [PATCH] ARM: Add TLB flushing for both entries in a PMD
> > >
> > > On Wed, Nov 23, 2011 at 10:21:37AM +0000, Changhwan Youn wrote:
> > > > I have tested this patch on several exynos machines which
> > > > have a9 cores and it worked fine.
> > > > Though I'm not sure that android boot and running simple applications
> > > > are enough test for this patch.
> > >
> > > Thanks for testing but the A9 would work fine without this patch. The
> > > problem is on A15 where level 1 page table entries (pgd) are cached by
> > > the TLB independently of level 2 entries (pte). The original code is
> > > only flushing one entry in level 1 rather than 2.
> >
> > Thank you for the answer.
> > The one thing I don't understand is why A9 works fine without this
> > patch. I know that A9 has worked fine without this patch.
> > It seems that without this patch, invalid VA->PA mapping can remains in TLB
> > and this can cause wrong PA access by user process.
> > Can you explain why there's no wrong PA access in A9?
> 
> This patch only fixes a bug in the pte_free_tlb() function, used when
> freeing the page tables and its goal is to make sure pmd level entries
> to not point to an already freed pte. Unmapping user VA->PA mappings
> happens at the pte level and page TLBs are flushed separately. If you
> look at the original code, it only adds a TLB flush for a single address
> while a pte table covers 2MB.
> 
> In theory, on A9 SMP there could also be an issue but given the timing
> and memory ordering conditions that's impossible in practice. Basically,
> the requirement is that clearing of the pmd entry on one CPU does not
> become visible to another CPU before the corresponding pte page has been
> re-used. The A9 only caches a page table walk in the TLB if there is a
> complete VA->PA translation.

Regards,

Changhwan