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@xxxxxxxxx>
---
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 a5930042139d3..bcca1c9b9a027 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 */
--
2.41.0
