[PATCH v12 7/7] arm64: armv8_deprecated: disable swp emulation when FEAT_LSUI present

Yeoreum Yun
2026-01-21T19:06:52Z

The purpose of supporting LSUI is to eliminate PAN toggling.
CPUs that support LSUI are unlikely to support a 32-bit runtime.
Since environments that support both LSUI and
a 32-bit runtimeare expected to be extremely rare,
not to emulate the SWP instruction using LSUI instructions
in order to remove PAN toggling, and instead simply disable SWP emulation.

Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
---
arch/arm64/kernel/armv8_deprecated.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/arch/arm64/kernel/armv8_deprecated.c b/arch/arm64/kernel/armv8_deprecated.c
index e737c6295ec7..049754f7da36 100644
--- a/arch/arm64/kernel/armv8_deprecated.c
+++ b/arch/arm64/kernel/armv8_deprecated.c
@@ -610,6 +610,22 @@ static int __init armv8_deprecated_init(void)
}

#endif
+
+#ifdef CONFIG_SWP_EMULATION
+ /*
+ * The purpose of supporting LSUI is to eliminate PAN toggling.
+ * CPUs that support LSUI are unlikely to support a 32-bit runtime.
+ * Since environments that support both LSUI and a 32-bit runtime
+ * are expected to be extremely rare, we choose not to emulate
+ * the SWP instruction using LSUI instructions in order to remove PAN toggling,
+ * and instead simply disable SWP emulation.
+ */
+ if (cpus_have_final_cap(ARM64_HAS_LSUI)) {
+ insn_swp.status = INSN_UNAVAILABLE;
+ pr_info("swp/swpb instruction emulation is not supported on this system\n");
+ }
+#endif
+
for (int i = 0; i < ARRAY_SIZE(insn_emulations); i++) {
struct insn_emulation *ie = insn_emulations[i];

--
LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}