[PATCH] Document Linux's memory barriers [try #2]

[Paul Mackerras]

Alan Cox writes:

> On Wed, Mar 08, 2006 at 02:37:58PM +0000, David Howells wrote:
> > + (*) reads can be done speculatively, and then the result discarded should it
> > +     prove not to be required;
> 
> That might be worth an example with an if() because PPC will do this and if 
> its a read with a side effect (eg I/O space) you get singed..

On PPC machines, the PTE has a bit called G (for Guarded) which
indicates that the memory mapped by it has side effects.  It prevents
the CPU from doing speculative accesses (i.e. the CPU can't send out a
load from the page until it knows for sure that the program will get
to that instruction) and from prefetching from the page.

The kernel sets G=1 on MMIO and PIO pages in general, as you would
expect, although you can get G=0 mappings for framebuffers etc. if you
ask specifically for that.

Paul.