[xfs-masters] Re: Interaction between Xen and XFS: stray RW mappings

[Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>]

On Mon, 2007-10-15 at 13:07 +0200, Andi Kleen wrote:
> On Tue, Oct 16, 2007 at 12:56:46AM +1000, Nick Piggin wrote:
> > Is this true even if you don't write through those old mappings?
> 
> I think it happened for reads too.  It is a little counter intuitive
> because in theory the CPU doesn't need to write back non dirty lines,
> but in the one case which took so long to debug exactly this happened
> somehow.

The problem exist also on ppc, and afaik, is due to the line being in
the cache at all (either dirty (write) or not (read)), thus causing the
snoop logic to hit, that is what's causing the problem vs. non cached
accesses.

Also, on some processors, the simple fact of having the page mapped can
cause the CPU to prefetch from it even if it's not actually accessed
(speculative prefetch can cross page boundaries if things are mapped).

Ben.