USB mass storage and ARM cache coherency

[Catalin Marinas]

On Sat, 2010-03-06 at 19:36 +0000, Russell King - ARM Linux wrote:
> On Sat, Mar 06, 2010 at 04:17:23PM +0530, James Bottomley wrote:
> > On a fault in of exec data, we first try to get the page out of the page
> > cache.  If it's not present, we put the faulting process to sleep and
> > fetch it in from storage.  When we do the read, on the PIO path, the
> > kernel alias for the page becomes dirty.  Some time later, we place the
> > page into the user space (updating the pte entry that caused a fault).
> > At this point, we'll call both flush_icache_page() and
> > update_mmu_cache() ... this is where the I/D resolution should be done.
> 
> No - this is where things get extremely icky.
> 
> The problem at this point occurs on SMP architectures.  As soon as you
> update the PTE entry, it is visible to other threads of the application.
> If you do I-cache handling after updating the PTE, then there is a window
> where another CPU can execute the page:
> 
> CPU0                    CPU1
>                         speculatively prefetches from page N via kernel
>                         mapping, loads garbage into I-cache
> attempts to execute P
> page fault
> page N allocated
> set_pte_at
>                         executes P
>                         *splat*
> flush I-cache

You have two choices - either invalidate the I-cache before the user pte
becomes visible or set the page as not-executable in set_pte_at() and
later mark it as executable in update_mmu_cache (via set_pte_ext).

We currently invalidate the whole I-cache for historical reasons but we
could actually only invalidate a single page. Since even on latest ARM
CPUs, the I-cache is a real VIPT (i.e. can have aliases), we would need
to invalidate on the user mapping (or create a temporary one). The
latter approach of clearing the X bit in set_pte_at may actually help
with this scenario (I haven't done any tests though).

-- 
Catalin