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bpf, x64: Comment tail_call_cnt initialisation

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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>
This commit is contained in:
Leon Hwang 2023-09-12 23:04:40 +08:00 committed by Alexei Starovoitov
parent 96daa98742
commit 2bee9770f3
1 changed files with 4 additions and 0 deletions
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4
arch/x86/net/bpf_jit_comp.c
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@ -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 */