From ce0761419faefbe9e450749ccc879ff88843af12 Mon Sep 17 00:00:00 2001 From: "Naveen N. Rao" Date: Sat, 24 Sep 2016 02:05:01 +0530 Subject: 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 Signed-off-by: Michael Ellerman --- arch/powerpc/include/asm/ppc-opcode.h | 2 ++ 1 file changed, 2 insertions(+) (limited to 'arch/powerpc/include/asm') diff --git a/arch/powerpc/include/asm/ppc-opcode.h b/arch/powerpc/include/asm/ppc-opcode.h index 127ebf5862b4..54ff8ce7fa96 100644 --- a/arch/powerpc/include/asm/ppc-opcode.h +++ b/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 -- cgit v1.2.1