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. - To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
