Messages in this thread |  | | | Date | Thu, 9 Mar 2006 09:01:57 +1100 | | From | Paul Mackerras <> | | 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.
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