Re: Interaction between Xen and XFS: stray RW mappings

Andi Kleen
2007-10-22T09:49:18Z

On Mon, Oct 22, 2007 at 08:16:01AM +1000, Benjamin Herrenschmidt wrote:
> 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.

That makes sense. Snoop can effectively turn a read into a write.

> 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).

Exactly that happens on x86. Normally prefetches stop on TLB miss,
but the CPU can do speculative TLB fetches too.

Also even without any prefetching the CPU does speculative execution
and that can lead to random addresses being followed.

-Andi