Unnecessary cache-line flush on page table updates ?

[Catalin Marinas]

On Tue, Jul 05, 2011 at 12:20:19AM +0100, Russell King - ARM Linux wrote:
> Please can you also check whether BTC invalidation is required when a
> page is removed from the page tables for the purpose of swapping it
> out (so the PTE entry becomes zero).

Not required.

> Also, is BTC invalidation required if the very same page that was
> there before is later restored without any modification to the
> instruction content of that page.  (I'm thinking page is aged to old,
> and so unmapped, then a prefetch abort which reinstates the same page
> that was there previously.)

Not required.

> Finally, is BTC invalidation required if a different physical page
> containing the same instruction content as before is placed at that
> location?

Required (see the comment below).

> Lastly, please check whether population and removal of page table
> entries for NX pages require BTC invalidation.  I expect not.
 
Not required.

Basically the rules for BTC are similar to those for the I-cache, but it
can be either ASID-tagged VIVT or VIPT, independent of the type of the
I-cache. I think current implementations of the BTC are ASID-tagged VIVT
but the architecture doesn't mandate this.

So the mapping of a data page doesn't require BTC invalidation, even if
the page is not marked as XN.

Having a branch between BTC and ISB is OK as long as we don't change the
mapping of the branch.

Mapping a page in a previously unmapped location doesn't generally
require BTC invalidation if we guarantee that there are no BTC entries
for that location (IOW, we got a new ASID or unmapping the page
previously had a BTC invalidation).

-- 
Catalin