arm64: Introduce scaffolding to add ACTLR_EL1 to thread state

Some CPUs expose IMPDEF features in ACTLR_EL1 that can be meaningfully
controlled per-thread (like TSO control on Apple cores). Add the basic
scaffolding to save/restore this register as part of context switching.

This mechanism is disabled by default both by config symbol and via a
runtime check, which ensures it is never triggered unless the system is
known to need it for some feature (which also implies that the layout of
ACTLR_EL1 is uniform between all CPU core types).

Signed-off-by: Hector Martin <[email protected]>

Author: marcan

arch/arm64/Kconfig

@@ -380,6 +380,9 @@ config KASAN_SHADOW_OFFSET
 config UNWIND_TABLES
 	bool
 
+config ARM64_ACTLR_STATE
+	bool
+
 source "arch/arm64/Kconfig.platforms"
 
 menu "Kernel Features"

arch/arm64/include/asm/cpufeature.h

@@ -915,6 +915,11 @@ static inline unsigned int get_vmid_bits(u64 mmfr1)
 	return 8;
 }
 
+static __always_inline bool system_has_actlr_state(void)
+{
+	return false;
+}
+
 struct arm64_ftr_reg *get_arm64_ftr_reg(u32 sys_id);
 
 extern struct arm64_ftr_override id_aa64mmfr1_override;

arch/arm64/include/asm/processor.h

@@ -179,6 +179,9 @@ struct thread_struct {
 	u64			sctlr_user;
 	u64			svcr;
 	u64			tpidr2_el0;
+#ifdef CONFIG_ARM64_ACTLR_STATE
+	u64			actlr;
+#endif
 };
 
 static inline unsigned int thread_get_vl(struct thread_struct *thread,

arch/arm64/kernel/process.c

@@ -374,6 +374,9 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 		if (system_supports_tpidr2())
 			p->thread.tpidr2_el0 = read_sysreg_s(SYS_TPIDR2_EL0);
 
+		if (system_has_actlr_state())
+			p->thread.actlr = read_sysreg(actlr_el1);
+
 		if (stack_start) {
 			if (is_compat_thread(task_thread_info(p)))
 				childregs->compat_sp = stack_start;
@@ -516,6 +519,19 @@ void update_sctlr_el1(u64 sctlr)
 	isb();
 }
 
+/*
+ * IMPDEF control register ACTLR_EL1 handling. Some CPUs use this to
+ * expose features that can be controlled by userspace.
+ */
+static void actlr_thread_switch(struct task_struct *next)
+{
+	if (!system_has_actlr_state())
+		return;
+
+	current->thread.actlr = read_sysreg(actlr_el1);
+	write_sysreg(next->thread.actlr, actlr_el1);
+}
+
 /*
  * Thread switching.
  */
@@ -533,6 +549,7 @@ struct task_struct *__switch_to(struct task_struct *prev,
 	ssbs_thread_switch(next);
 	erratum_1418040_thread_switch(next);
 	ptrauth_thread_switch_user(next);
+	actlr_thread_switch(next);
 
 	/*
 	 * Complete any pending TLB or cache maintenance on this CPU in case

arch/arm64/kernel/setup.c

@@ -380,6 +380,14 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
 	 */
 	init_task.thread_info.ttbr0 = phys_to_ttbr(__pa_symbol(reserved_pg_dir));
 #endif
+#ifdef CONFIG_ARM64_ACTLR_STATE
+	/* Store the boot CPU ACTLR_EL1 value as the default. This will only
+	 * be actually restored during context switching iff the platform is
+	 * known to use ACTLR_EL1 for exposable features and its layout is
+	 * known to be the same on all CPUs.
+	 */
+	init_task.thread.actlr = read_sysreg(actlr_el1);
+#endif
 
 	if (boot_args[1] || boot_args[2] || boot_args[3]) {
 		pr_err("WARNING: x1-x3 nonzero in violation of boot protocol:\n"