[PATCH] x86/bpf: Avoid emitting LOCK prefix for XCHG atomic ops

[Uros Bizjak]

The x86 XCHG instruction is implicitly locked when one of the
operands is a memory location, making an explicit LOCK prefix
unnecessary.

Stop emitting the LOCK prefix for BPF_XCHG in the JIT atomic
read-modify-write helpers. This avoids redundant instruction
prefixes while preserving correct atomic semantics.

No functional change for other atomic operations.

Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
---
 arch/x86/net/bpf_jit_comp.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index b69dc7194e2c..77d724525808 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1305,7 +1305,8 @@ static int emit_atomic_rmw(u8 **pprog, u32 atomic_op,
 {
 	u8 *prog = *pprog;
 
-	EMIT1(0xF0); /* lock prefix */
+	if (atomic_op != BPF_XCHG)
+		EMIT1(0xF0); /* lock prefix */
 
 	maybe_emit_mod(&prog, dst_reg, src_reg, bpf_size == BPF_DW);
 
@@ -1347,7 +1348,9 @@ static int emit_atomic_rmw_index(u8 **pprog, u32 atomic_op, u32 size,
 {
 	u8 *prog = *pprog;
 
-	EMIT1(0xF0); /* lock prefix */
+	if (atomic_op != BPF_XCHG)
+		EMIT1(0xF0); /* lock prefix */
+
 	switch (size) {
 	case BPF_W:
 		EMIT1(add_3mod(0x40, dst_reg, src_reg, index_reg));
-- 
2.52.0