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/include/asm/ppc-opcode.h

@@ -236,6 +236,7 @@
 #define PPC_INST_STWU			0x94000000
 #define PPC_INST_MFLR			0x7c0802a6
 #define PPC_INST_MTLR			0x7c0803a6
+#define PPC_INST_MTCTR			0x7c0903a6
 #define PPC_INST_CMPWI			0x2c000000
 #define PPC_INST_CMPDI			0x2c200000
 #define PPC_INST_CMPW			0x7c000000
@@ -250,6 +251,7 @@
 #define PPC_INST_SUB			0x7c000050
 #define PPC_INST_BLR			0x4e800020
 #define PPC_INST_BLRL			0x4e800021
+#define PPC_INST_BCTR			0x4e800420
 #define PPC_INST_MULLD			0x7c0001d2
 #define PPC_INST_MULLW			0x7c0001d6
 #define PPC_INST_MULHWU			0x7c000016