On Thu, 23 Aug 2007, Jesse Barnes wrote: > On Thursday, August 23, 2007 12:27 am Benjamin Herrenschmidt wrote: > > > Of course, the normal memory barrier would usually be a > > > "spin_unlock()" or something like that, not a "wmb()". In fact, I > > > don't think the powerpc implementation (as an example of this) will > > > actually synchronize with anything *but* a spin_unlock(). > > > > We are even more sneaky in the sense that we set a per-cpu flag on > > any MMIO write and do the sync automatically in spin_unlock() :-) > > Yeah, that's a reasonable thing to do, and in fact I think there's code > to do something similar when a task is switched out (this keeps user > level drivers from having do mmiowb() type things). Yes there is, git commit e08e6c521355cd33e647b2f739885bc3050eead6. On SN2 any user process performing memory-mapped IO directly to a device needs something like mmiowb() to be performed at the node of the CPU it last ran on when the task context switches onto a new CPU. The current code performs this action for all inter-CPU context switches, but we had discussed the possibility of targetting the action only when the user process has actually mapped a device for IO. I believe it was decided that this level of complexity wasn't warranted unless this simple solution was found to cause a problem. That reminds me. Are the people who are working on the user-level driver effort including a capability similar to mmiowb()? If we had that capability we could eventually do away with the change mentioned above. But that would come after all user-level drivers were coded to include the mmiowb()-like calls, and existing drivers which provide mmap() capability directly to hardware go away. Brent -- Brent Casavant All music is folk music. I ain't bcasavan@xxxxxxx never heard a horse sing a song. Silicon Graphics, Inc. -- Louis Armstrong - To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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