djprobes status

[Ingo Molnar]

On Fri, 2006-09-15 at 20:50 +0200, Ingo Molnar wrote:
> Am i missing some difficulty? The performance of probe
> insertion/removal
> does not really matter, the critical path is probe execution.

a third possibility would be to generate not a jump straight into the
trampoline, but a jump to a kprobes-controlled function:

	pushw $target_IP
	ret

this makes the inserted code 6 bytes, but gives full control to kprobes:
you can start a probe in a non-patched function, which would start by
first disabling preemption. It's almost as fast as the direct jump:
modern CPUs are able to optimize such ret sequences just fine.

at the end of the DCR trampoline you'd not jump back straight to the
code, but you'd return-jump to another global kprobes function: which
would re-enable preemption and which would jump back to the next
instruction.

this is all still infinitely faster than INT3 + single-stepping.

this would make probe installation and removal quite simple, at the
expense of some minimal runtime overhead. (10 cycles at most, on modern
CPUs)

	Ingo