USB mass storage and ARM cache coherency

[benh@kernel.crashing.org (Benjamin Herrenschmidt)]

On Sun, 2010-03-07 at 09:07 +0530, James Bottomley wrote:
> So, assuming full congruence of user space, can't you use the VMA as an
> indicator?  i.e. if we have no user space mappings, we have to flush the
> icache ... if we have one or more, the icache has been flushed and
> placing the same page congruently in a different address space benefits
> from that prior flush, so consequently there's no need to flush again?

the VMA ? Or you mean struct page -> mapping ? That would work I suppose
in the case where we want to flush the icache pages for all pages mapped
into user space. But on processors that support per-page execute
permission, we really only want to flush pages that are executed from
(lazily). In that case, we do need a dedicated bit to keep track of
whether a given page has been flushed already.

> I also think we've established the relevant facts for the I/O thread
> (that we only need to either flush the kernel D cache or mark it as to
> be flushed later on PIO reads).  We're now into deep technicalities of
> how the mm system operates at the architecture level, so perhaps we
> should move this to linux-arch? 

No objection though moving threads after the fact is a recipe for
trouble :-)

Cheers,
Ben.