From 2bee9770f3c6be736a28725cb0f93775ed22e720 Mon Sep 17 00:00:00 2001 From: Leon Hwang Date: Tue, 12 Sep 2023 23:04:40 +0800 Subject: [PATCH] bpf, x64: Comment tail_call_cnt initialisation Without understanding emit_prologue(), it is really hard to figure out where does tail_call_cnt come from, even though searching tail_call_cnt in the whole kernel repo. By adding these comments, it is a little bit easier to understand tail_call_cnt initialisation. Signed-off-by: Leon Hwang Link: https://lore.kernel.org/r/20230912150442.2009-2-hffilwlqm@gmail.com Signed-off-by: Alexei Starovoitov --- arch/x86/net/bpf_jit_comp.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c index a5930042139d..bcca1c9b9a02 100644 --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -303,8 +303,12 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf, prog += X86_PATCH_SIZE; if (!ebpf_from_cbpf) { if (tail_call_reachable && !is_subprog) + /* When it's the entry of the whole tailcall context, + * zeroing rax means initialising tail_call_cnt. + */ EMIT2(0x31, 0xC0); /* xor eax, eax */ else + /* Keep the same instruction layout. */ EMIT2(0x66, 0x90); /* nop2 */ } EMIT1(0x55); /* push rbp */