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    SubjectRe: USB mass storage and ARM cache coherency
    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.


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