powerpc/kprobes: Implement Optprobes

Current infrastructure of kprobe uses the unconditional trap instruction
to probe a running kernel. Optprobe allows kprobe to replace the trap
with a branch instruction to a detour buffer. Detour buffer contains
instructions to create an in memory pt_regs. Detour buffer also has a
call to optimized_callback() which in turn call the pre_handler(). After
the execution of the pre-handler, a call is made for instruction
emulation. The NIP is determined in advanced through dummy instruction
emulation and a branch instruction is created to the NIP at the end of
the trampoline.

To address the limitation of branch instruction in POWER architecture,
detour buffer slot is allocated from a reserved area. For the time
being, 64KB is reserved in memory for this purpose.

Instructions which can be emulated using analyse_instr() are the
candidates for optimization. Before optimization ensure that the address
range between the detour buffer allocated and the instruction being
probed is within +/- 32MB.

Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/code-patching.h

@@ -35,6 +35,7 @@ int instr_is_branch_to_addr(const unsigned int *instr, unsigned long addr);
 unsigned long branch_target(const unsigned int *instr);
 unsigned int translate_branch(const unsigned int *dest,
 			      const unsigned int *src);
+extern bool is_conditional_branch(unsigned int instr);
 #ifdef CONFIG_PPC_BOOK3E_64
 void __patch_exception(int exc, unsigned long addr);
 #define patch_exception(exc, name) do { \