On Fri, 2012-12-07 at 09:22 +0000, Jon Medhurst (Tixy) wrote: > On Thu, 2012-12-06 at 14:19 -0500, Steven Rostedt wrote: > > Hmm, your use of "may or may not" seems as you may not know this answer. > > I wonder if you can use the break point method as x86 does now, and > > remove the stop machine completely. Basically this is how it works: > > > > add sw breakpoints to all locations to modify (the bp handler just does > > a nop over the instruction). > > > > send an IPI to all CPUs to flush their icache. > > > > Modify the non breakpoint part of the instruction with the new > > instruction. > > > > send an IPI to all CPUs to flush their icache > > > > Replace the breakpoint with the finished instruction. > > If I understand correctly then this method can't work on ARM because a > 'software breakpoint' is 'replace an instruction with a known undefined > instruction _of the same size_'. It haa to be the same size because code > like this: > > it eq /* If condition code 'eq' true */ > insA /* then execute this instruction */ > insB /* Always execute this */ > > if we replace insA with a breakpoint which is shorter, then we have > > it eq /* If condition code 'eq' true */ > bkpt /* then execute the breakpoint */ > insA-part2 /* Always execute this garbage */
Why always execute the garbage? Do what we do in x86, where the breakpoint is only 1 byte and the instruction being replaced is 5 bytes. The breakpoint handler returns to the instruction after the "garbage" (insB). > insB /* Always execute this */ > > and to complicate matters more, the 'it' instruction can make up to the > next four instructions conditional, so you can't reverse decode the > instruction stream reliably to even detect such code. > > And further, it's implementation defined (up to who every creates the > silicon) whether an undefined instructions actually causes an abort when > it occurs in such an 'it' block, it may just execute as a nop. > > Welcome to the work of ARM :-) > But also realize that function tracing is special :-) We have no cases like this. The instruction being replaced is a call to mcount. In fact, we replace it at boot with a nop. And this method only replaces that nop into a call to function tracer, or replaces the call to function tracer back to a nop. Always at the start of the function, and never involved with conditionals. This limitation that function tracing imposes on what we replace makes things a bit more sane in how we replace it. -- Steve -- 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/
