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Peter Anvin" Subject: [PATCH] x86/bpf: Avoid emitting LOCK prefix for XCHG atomic ops Date: Mon, 8 Dec 2025 17:33:34 +0100 Message-ID: <20251208163420.7643-1-ubizjak@gmail.com> X-Mailer: git-send-email 2.52.0 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 Cc: Alexei Starovoitov Cc: Daniel Borkmann Cc: Andrii Nakryiko Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Borislav Petkov Cc: Dave Hansen Cc: "H. Peter Anvin" --- 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