USB mass storage and ARM cache coherency

[Russell King - ARM Linux]

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.