arm64: armv8_deprecated: Disable swp emulation when FEAT_LSUI present

The purpose of supporting LSUI is to eliminate PAN toggling. CPUs that
support LSUI are unlikely to support a 32-bit runtime. Rather than
emulating the SWP instruction using LSUI instructions in order to remove
PAN toggling, simply disable SWP emulation.

Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
[catalin.marinas@arm.com: some tweaks to the in-code comment]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: LeviYeoReum

arch/arm64/kernel/armv8_deprecated.c

@@ -610,6 +610,20 @@ 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. Rather
+	 * than emulating the SWP instruction using LSUI instructions, 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];