Messages in this thread | | | Subject | Re: [PATCH] ARM: ftrace: Ensure code modifications are synchronised across all cpus | From | Steven Rostedt <> | Date | Fri, 07 Dec 2012 11:36:40 -0500 |
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On Fri, 2012-12-07 at 16:23 +0000, Russell King - ARM Linux wrote:
> That's fine if there are better ways. If your view is that this would > bring things "up to the future" consider this: what you suggest is possible > with the standard ARM 32-bit instruction set. With the more modern Thumb > instruction set, because we now effectively have prefixes, where those > prefixes control the execution of the following instructions, what you > suggest becomes no longer possible. > > So, it's not a question of bringing stuff up to the future at all... you > can call it a design regression of you will, but you're really making > demands about how CPUs work which are outside of your remit. > > Think of this a bit like you changing the opcodes immediately following a > 'LOCK' prefix on x86. I suspect divorsing the following opcodes from its > prefix would be very bad for the instructions atomicity.
But what about the limitations that the function tracer imposes on the code that gets modified by stop_machine()?
1) the original code is simply a call to mcount
2) on boot up, that call gets converted into a nop
3) the code that gets changed will only be converting a nop to a call into the function tracer, and back again.
IOW, it's a very limited subset of the ARM assembly that gets touched. I'm not sure what the op codes are for the above, but I can imagine they don't impose the prefixes as you described.
If that's the case, is it still possible to change to the breakpoint method?
-- Steve
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