Commit e09f6be4 authored by Uros Bizjak's avatar Uros Bizjak Committed by Alexei Starovoitov
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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: default avatarUros 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: default avatarAlexei Starovoitov <ast@kernel.org>
parent c2f2f005

arch/x86/net/bpf_jit_comp.c

+5 −2
Original line number Diff line number Diff line
@@ -1305,6 +1305,7 @@ static int emit_atomic_rmw(u8 **pprog, u32 atomic_op, 
{

	u8 *prog = *pprog;



	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;



	if (atomic_op != BPF_XCHG)

		EMIT1(0xF0); /* lock prefix */



	switch (size) {

	case BPF_W:

		EMIT1(add_3mod(0x40, dst_reg, src_reg, index_reg));