powerpc/bpf: Implement support for tail calls

Tail calls allow JIT'ed eBPF programs to call into other JIT'ed eBPF
programs. This can be achieved either by:
(1) retaining the stack setup by the first eBPF program and having all
subsequent eBPF programs re-using it, or,
(2) by unwinding/tearing down the stack and having each eBPF program
deal with its own stack as it sees fit.

To ensure that this does not create loops, there is a limit to how many
tail calls can be done (currently 32). This requires the JIT'ed code to
maintain a count of the number of tail calls done so far.

Approach (1) is simple, but requires every eBPF program to have (almost)
the same prologue/epilogue, regardless of whether they need it. This is
inefficient for small eBPF programs which may not sometimes need a
prologue at all. As such, to minimize impact of tail call
implementation, we use approach (2) here which needs each eBPF program
in the chain to use its own prologue/epilogue. This is not ideal when
many tail calls are involved and when all the eBPF programs in the chain
have similar prologue/epilogue. However, the impact is restricted to
programs that do tail calls. Individual eBPF programs are not affected.

We maintain the tail call count in a fixed location on the stack and
updated tail call count values are passed in through this. The very
first eBPF program in a chain sets this up to 0 (the first 2
instructions). Subsequent tail calls skip the first two eBPF JIT
instructions to maintain the count. For programs that don't do tail
calls themselves, the first two instructions are NOPs.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/net/bpf_jit.h

@@ -40,6 +40,8 @@
 #define PPC_BLR()		EMIT(PPC_INST_BLR)
 #define PPC_BLRL()		EMIT(PPC_INST_BLRL)
 #define PPC_MTLR(r)		EMIT(PPC_INST_MTLR | ___PPC_RT(r))
+#define PPC_BCTR()		EMIT(PPC_INST_BCTR)
+#define PPC_MTCTR(r)		EMIT(PPC_INST_MTCTR | ___PPC_RT(r))
 #define PPC_ADDI(d, a, i)	EMIT(PPC_INST_ADDI | ___PPC_RT(d) |	      \
 				     ___PPC_RA(a) | IMM_L(i))
 #define PPC_MR(d, a)		PPC_OR(d, a, a)