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

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@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20251208163420.7643-1-ubizjak@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index b0bac2a66eff3c471d6f4e1af1936ea8a2a2d763..e3b1c4b1d550419a6b9aab8c11acbe433b3e3a5a 100644 (file)
--- 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));