Commit 2bee9770 authored Sep 12, 2023 by Leon Hwang Committed by Alexei Starovoitov Sep 12, 2023

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 <hffilwlqm@gmail.com>
Link: https://lore.kernel.org/r/20230912150442.2009-2-hffilwlqm@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>

parent 96daa987

arch/x86/net/bpf_jit_comp.c

+4 −0

Original line number	Diff line number	Diff line
		@@ -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 */