[PATCH bpf-next 1/3] bpf, x64: Comment tail_call_cnt initialisation

From: Leon Hwang <hffilwlqm@gmail.com>
To: bpf@vger.kernel.org
Cc: ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org,
	maciej.fijalkowski@intel.com, song@kernel.org, iii@linux.ibm.com,
	xukuohai@huawei.com, hffilwlqm@gmail.com,
	kernel-patches-bot@fb.com
Subject: [PATCH bpf-next 1/3] bpf, x64: Comment tail_call_cnt initialisation
Date: Tue, 12 Sep 2023 23:04:40 +0800	[thread overview]
Message-ID: <20230912150442.2009-2-hffilwlqm@gmail.com> (raw)
In-Reply-To: <20230912150442.2009-1-hffilwlqm@gmail.com>

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>
---
 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

