Re: USB mass storage and ARM cache coherency

From: Catalin Marinas
Date: Thu Mar 04 2010 - 10:32:41 EST

On Thu, 2010-03-04 at 14:27 +0000, Russell King - ARM Linux wrote:
> On Thu, Mar 04, 2010 at 07:51:52PM +0530, James Bottomley wrote:
> > On Thu, 2010-03-04 at 14:51 +0100, Pavel Machek wrote:
> > > Seems like ARM has requirement other architectures do not, that is
> > > a) not documented anywhere
> > > b) causes problems
> > >
> > > You could argue that performance improvement (how big is it, anyway?)
> > > is worth it, but this should be agreed to by wider community...
> >
> > Performance is always worth it provided we don't sacrifice correctness.
> > The thing which was discovered in this thread is basically that ARM is
> > handling deferred flushing (for D/I coherency) in a slightly different
> > way from everyone else ... once that's fixed, ARM will likely not have
> > the D/I problem, but we'll still have the libata (and other PIO systems)
> > D flushing issue.
> I think you've got that backwards.
> Reversing the meaning of PG_arch_1 will probably fix the D aliasing issue -
> since we'll interpret '0' to mean "page is dirty, it needs flushing before
> hitting userspace", whereas '1' means "page has been cleaned; there are no
> aliases."
> This doesn not address the I/D coherency issue, where the Icache needs
> attention to get rid of speculatively loaded cache lines while old data
> was present in the cache.

The I-cache flushing is already handled in update_mmu_cache (or
set_pte_at in a future patch; I'm not talking about other issues on
ARM11MPCore here).

We always invalidate the I-cache currently (since we may have DMA
transfers and the page's D-cache is clean). As an optimisation, we could
use PG_arch_2 for I-cache but I don't think there is much performance
benefit compared to always invalidating the I-cache flushing.

My understanding from this long discussion is that we cannot get the
kernel modifying a page cache page which is already mapped in user space
(well, ptrace does this but we flush the cache there already).


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