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

[David S. Miller]

From: Paul Mackerras <paulus at samba.org>
Date: Thu, 9 Mar 2006 09:01:57 +1100

> 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.

Sparc64 has a similar PTE bit called "E" for "side-Effect".
And we also do the same thing as powerpc for framebuffers.

Note that on sparc64 in our asm/io.h PIO/MMIO accessor macros
we use physical addresses, so we don't have to map anything
in ioremap(), and use a special address space identifier on
the loads and stores that indicates "E" behavior is desired.