Back when all of the callee-saved registers where always pushed to stack in x64 JIT prologue, tail call counter was placed at the bottom of the BPF program's stack frame that had a following layout: +-------------+ | ret addr | +-------------+ | rbp | <- rbp +-------------+ | | | free space | | from: | | sub $x,%rsp | | | +-------------+ | rbx | +-------------+ | r13 | +-------------+ | r14 | +-------------+ | r15 | +-------------+ | tail call | <- rsp | counter | +-------------+ In order to restore the callee saved registers, epilogue needed to explicitly toss away the tail call counter via "pop %rbx" insn, so that %rsp would be back at the place where %r15 was stored. Currently, the tail call counter is placed on stack *before* the callee saved registers (brackets on rbx through r15 mean that they are now pushed to stack only if they are used): +-------------+ | ret addr | +-------------+ | rbp | <- rbp +-------------+ | | | free space | | from: | | sub $x,%rsp | | | +-------------+ | tail call | | counter | +-------------+ ( rbx ) +-------------+ ( r13 ) +-------------+ ( r14 ) +-------------+ ( r15 ) <- rsp +-------------+ For the record, the epilogue insns consist of (assuming all of the callee saved registers are used by program): pop %r15 pop %r14 pop %r13 pop %rbx pop %rcx leaveq retq "pop %rbx" for getting rid of tail call counter was not an option anymore as it would overwrite the restored value of %rbx register, so it was changed to use the %rcx register. Since epilogue can start popping the callee saved registers right away without any additional work, the "pop %rcx" could be dropped altogether as "leave" insn will simply move the %rbp to %rsp. IOW, tail call counter does not need the explicit handling. Having in mind the explanation above and the actual reason for that, let's piggy back on "leave" insn for discarding the tail call counter from stack and remove the "pop %rcx" from epilogue. Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@xxxxxxxxx> --- arch/x86/net/bpf_jit_comp.c | 2 -- 1 file changed, 2 deletions(-) diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c index 26f43279b78b..a263918043ce 100644 --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -1441,8 +1441,6 @@ xadd: if (is_imm8(insn->off)) /* Update cleanup_addr */ ctx->cleanup_addr = proglen; pop_callee_regs(&prog, callee_regs_used); - if (tail_call_reachable) - EMIT1(0x59); /* pop rcx, get rid of tail_call_cnt */ EMIT1(0xC9); /* leave */ EMIT1(0xC3); /* ret */ break; -- 2.20.1
