On Mon, Dec 10, 2012 at 01:02:17PM +0000, Steven Rostedt wrote: > On Mon, 2012-12-10 at 10:04 +0000, Will Deacon wrote: > > Hi Jon, > > > > Back-pedalling a bit here, but I'm confused by one of your points below: > > > > On Fri, Dec 07, 2012 at 05:45:47PM +0000, Jon Medhurst (Tixy) wrote: > > > On Fri, 2012-12-07 at 12:13 -0500, Steven Rostedt wrote: > > > > I'll make my question more general: > > > > > > > > If I have a nop, that is a size of a call (branch and link), which is > > > > near the beginning of a function and not part of any conditional, and I > > > > want to convert it into a call (branch and link), would adding a > > > > breakpoint to it, modifying it to the call, and then removing the > > > > breakpoint be possible? Of course it would require syncing in between > > > > steps, but my question is, if the above is possible on a thumb2 ARM > > > > processor? > > > > > > I believe so. The details are (repeating your earlier explanation) ... > > > > > > 1. Replace first half of nop with 16bit 'breakpoint' instruction. > > > > Sort of -- you'd actually need 2x16-bit nops to make this work. > > Why?
Because the architecture doesn't provide any guarantees about concurrent modification of 32-bit nop instructions. If you stop the world every time, fine, but that's what we're trying to avoid, right? > > > However, wouldn't we need any of this breakpoint malarkey, why not just > > > just use a 16-bit branch instruction which branches over the second half > > > of the nop? :-) > > > > Yes, and I think if you do use two 16-bit nops, you can even get rid of all > > the intermediate `sync' operations (I guess you might want one at the end if > > you want the call to become visible at a particular point). > > Wont work. We are replacing a 32bit call with a nop. That nop must also > be 32bits, because we could eventually replace the nop(s) with a 32bit > call. Basically, we can never allow the second 16bit part ever be the > next instruction. If the first 16bit nop is executed, and then the task > gets preempted. The nops get converted to a 32bit call. The task gets > scheduled again and now is executing the second 16bits of the 32bit call > and we get unexpected (probably crashing) results. Damn, I didn't realise you wanted to put the 32-bit call back on pre-emption. Still, the `sync' is not needed when patching in a b for a nop. > By having either a 16bit breakpoint whose handler returns after the > second 16bit part, or a 16bit jump that simply jumps over the second > half, then all this should work. When the CPU processes a 32bit > instruction, it either processes all or non of it, correct? If you have two 16-bit nops, patching the first to branch over the second will work. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
