The point is to provide a way to dynamically enable code at runtime
without noticeable performance impact on the system. It's principally
useful to control the markers in the kernel, which can be placed in very
frequently executed code paths. The original markers add a memory read,
test and conditional branch at each marker site. By using the immediate
values patchset, it goes down to a load immediate value, test and branch.
However, Ingo was still unhappy with the conditional branch, so I cooked
this jump patching optimization on top of the immediate values. It
looks for an expected pattern which limits the liveliness of the %al and
ZF registers to the 3 instructions and, if it finds it, patches a jump
located just before the mov instruction to skip the whole pattern and
behave exactly like the conditional branch.
So basically we get code dynamically actvated by patching a single jump.