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fd4363fff3
Introduce a method for run-time instruction patching on a live SMP kernel
based on int3 breakpoint, completely avoiding the need for stop_machine().
The way this is achieved:
- add a int3 trap to the address that will be patched
- sync cores
- update all but the first byte of the patched range
- sync cores
- replace the first byte (int3) by the first byte of
replacing opcode
- sync cores
According to
http://lkml.indiana.edu/hypermail/linux/kernel/1001.1/01530.html
synchronization after replacing "all but first" instructions should not
be necessary (on Intel hardware), as the syncing after the subsequent
patching of the first byte provides enough safety.
But there's not only Intel HW out there, and we'd rather be on a safe
side.
If any CPU instruction execution would collide with the patching,
it'd be trapped by the int3 breakpoint and redirected to the provided
"handler" (which would typically mean just skipping over the patched
region, acting as "nop" has been there, in case we are doing nop -> jump
and jump -> nop transitions).
Ftrace has been using this very technique since
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