From:   Christophe Leroy <christophe.leroy@csgroup.eu>
To:     Michael Ellerman <mpe@ellerman.id.au>,
        Nicholas Piggin <npiggin@gmail.com>,
        "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
Cc:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        bpf@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>
Subject: [PATCH v2 2/9] powerpc/bpf/32: No need to zeroise r4 when not doing tail call
Date:   Wed,  1 Feb 2023 11:04:24 +0100
Message-Id: <9c5440b2b6d90a78600257433ac499b5c5101fbb.1675245773.git.christophe.leroy@csgroup.eu>
In-Reply-To: <4fd69ef7945518c3e27f96b95046a5c1468d35bf.1675245773.git.christophe.leroy@csgroup.eu>
References: <4fd69ef7945518c3e27f96b95046a5c1468d35bf.1675245773.git.christophe.leroy@csgroup.eu>
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r4 is cleared at function entry and used as tail call count.

But when the function does not perform tail call, r4 is
ignored, so no need to clear it.

Replace it by a NOP in that case.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 arch/powerpc/net/bpf_jit_comp32.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
index a379b0ce19ff..4e6caee9c98a 100644
--- a/arch/powerpc/net/bpf_jit_comp32.c
+++ b/arch/powerpc/net/bpf_jit_comp32.c
@@ -114,7 +114,10 @@ void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
 	int i;
 
 	/* Initialize tail_call_cnt, to be skipped if we do tail calls. */
-	EMIT(PPC_RAW_LI(_R4, 0));
+	if (ctx->seen & SEEN_TAILCALL)
+		EMIT(PPC_RAW_LI(_R4, 0));
+	else
+		EMIT(PPC_RAW_NOP());
 
 #define BPF_TAILCALL_PROLOGUE_SIZE	4
 
-- 
2.39.1