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Message-ID: <17423.21589.385336.68518@cargo.ozlabs.ibm.com>
Date: Thu, 9 Mar 2006 09:01:57 +1100
From: Paul Mackerras <paulus@samba.org>
To: Alan Cox <alan@redhat.com>
Cc: David Howells <dhowells@redhat.com>, torvalds@osdl.org, akpm@osdl.org,
       mingo@redhat.com, linux-arch@vger.kernel.org, linuxppc64-dev@ozlabs.org,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Document Linux's memory barriers [try #2]
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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.
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