arm64: armv8_deprecated: rework deprected instruction handling

Support for deprecated instructions can be enabled or disabled at
runtime. To handle this, the code in armv8_deprecated.c registers and
unregisters undef_hooks, and makes cross CPU calls to configure HW
support. This is rather complicated, and the synchronization required to
make this safe ends up serializing the handling of instructions which
have been trapped.

This patch simplifies the deprecated instruction handling by removing
the dynamic registration and unregistration, and changing the trap
handling code to determine whether a handler should be invoked. This
removes the need for dynamic list management, and simplifies the locking
requirements, making it possible to handle trapped instructions entirely
in parallel.

Where changing the emulation state requires a cross-call, this is
serialized by locally disabling interrupts, ensuring that the CPU is not
left in an inconsistent state.

To simplify sysctl management, each insn_emulation is given a separate
sysctl table, permitting these to be registered separately. The core
sysctl code will iterate over all of these when walking sysfs.

I've tested this with userspace programs which use each of the
deprecated instructions, and I've concurrently modified the support
level for each of the features back-and-forth between HW and emulated to
check that there are no spurious SIGILLs sent to userspace when the
support level is changed.

Signed-off-by: Mark Rutland <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: James Morse <[e-mail]>
Cc: Joey Gouly <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Will Deacon <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Will Deacon <[e-mail]>

arch/arm64/include/asm/traps.h

···
 
 struct pt_regs;
 
-struct undef_hook {
-	struct list_head node;
-	u32 instr_mask;
-	u32 instr_val;
-	u64 pstate_mask;
-	u64 pstate_val;
-	int (*fn)(struct pt_regs *regs, u32 instr);
-};
+#ifdef CONFIG_ARMV8_DEPRECATED
+bool try_emulate_armv8_deprecated(struct pt_regs *regs, u32 insn);
+#else
+static inline bool
+try_emulate_armv8_deprecated(struct pt_regs *regs, u32 insn)
+{
+	return false;
+}
+#endif /* CONFIG_ARMV8_DEPRECATED */
 
-void register_undef_hook(struct undef_hook *hook);
-void unregister_undef_hook(struct undef_hook *hook);
 void force_signal_inject(int signal, int code, unsigned long address, unsigned long err);
 void arm64_notify_segfault(unsigned long addr);
 void arm64_force_sig_fault(int signo, int code, unsigned long far, const char *str);

arch/arm64/kernel/armv8_deprecated.c

···
 enum legacy_insn_status {
 	INSN_DEPRECATED,
 	INSN_OBSOLETE,
+	INSN_UNAVAILABLE,
 };
 
 struct insn_emulation {
 	const char			*name;
-	struct list_head		node;
 	enum legacy_insn_status		status;
-	struct undef_hook		*hooks;
+	bool				(*try_emulate)(struct pt_regs *regs,
+						       u32 insn);
 	int				(*set_hw_mode)(bool enable);
+
 	int current_mode;
 	int min;
 	int max;
+
+	/*
+	 * sysctl for this emulation + a sentinal entry.
+	 */
+	struct ctl_table sysctl[2];
 };
 
 #define ARM_OPCODE_CONDTEST_FAIL   0
···
 	return ARM_OPCODE_CONDTEST_UNCOND;
 }
 
+#ifdef CONFIG_SWP_EMULATION
 /*
  *  Implement emulation of the SWP/SWPB instructions using load-exclusive and
  *  store-exclusive.
···
 	return 0;
 }
 
-/*
- * Only emulate SWP/SWPB executed in ARM state/User mode.
- * The kernel must be SWP free and SWP{B} does not exist in Thumb.
- */
-static struct undef_hook swp_hooks[] = {
-	{
-		.instr_mask	= 0x0fb00ff0,
-		.instr_val	= 0x01000090,
-		.pstate_mask	= PSR_AA32_MODE_MASK,
-		.pstate_val	= PSR_AA32_MODE_USR,
-		.fn		= swp_handler
-	},
-	{ }
-};
+static bool try_emulate_swp(struct pt_regs *regs, u32 insn)
+{
+	/* SWP{B} only exists in ARM state and does not exist in Thumb */
+	if (!compat_user_mode(regs) || compat_thumb_mode(regs))
+		return false;
+
+	if ((insn & 0x0fb00ff0) != 0x01000090)
+		return false;
+
+	return swp_handler(regs, insn) == 0;
+}
 
 static struct insn_emulation insn_swp = {
 	.name = "swp",
 	.status = INSN_OBSOLETE,
-	.hooks = swp_hooks,
+	.try_emulate = try_emulate_swp,
 	.set_hw_mode = NULL,
 };
+#endif /* CONFIG_SWP_EMULATION */
 
+#ifdef CONFIG_CP15_BARRIER_EMULATION
 static int cp15barrier_handler(struct pt_regs *regs, u32 instr)
 {
 	perf_sw_event(PERF_COUNT_SW_EMULATION_FAULTS, 1, regs, regs->pc);
···
 	return 0;
 }
 
-static struct undef_hook cp15_barrier_hooks[] = {
-	{
-		.instr_mask	= 0x0fff0fdf,
-		.instr_val	= 0x0e070f9a,
-		.pstate_mask	= PSR_AA32_MODE_MASK,
-		.pstate_val	= PSR_AA32_MODE_USR,
-		.fn		= cp15barrier_handler,
-	},
-	{
-		.instr_mask	= 0x0fff0fff,
-		.instr_val	= 0x0e070f95,
-		.pstate_mask	= PSR_AA32_MODE_MASK,
-		.pstate_val	= PSR_AA32_MODE_USR,
-		.fn		= cp15barrier_handler,
-	},
-	{ }
-};
+static bool try_emulate_cp15_barrier(struct pt_regs *regs, u32 insn)
+{
+	if (!compat_user_mode(regs) || compat_thumb_mode(regs))
+		return false;
+
+	if ((insn & 0x0fff0fdf) == 0x0e070f9a)
+		return cp15barrier_handler(regs, insn) == 0;
+
+	if ((insn & 0x0fff0fff) == 0x0e070f95)
+		return cp15barrier_handler(regs, insn) == 0;
+
+	return false;
+}
 
 static struct insn_emulation insn_cp15_barrier = {
 	.name = "cp15_barrier",
 	.status = INSN_DEPRECATED,
-	.hooks = cp15_barrier_hooks,
+	.try_emulate = try_emulate_cp15_barrier,
 	.set_hw_mode = cp15_barrier_set_hw_mode,
 };
+#endif /* CONFIG_CP15_BARRIER_EMULATION */
 
+#ifdef CONFIG_SETEND_EMULATION
 static int setend_set_hw_mode(bool enable)
 {
 	if (!cpu_supports_mixed_endian_el0())
···
 	return rc;
 }
 
-static struct undef_hook setend_hooks[] = {
-	{
-		.instr_mask	= 0xfffffdff,
-		.instr_val	= 0xf1010000,
-		.pstate_mask	= PSR_AA32_MODE_MASK,
-		.pstate_val	= PSR_AA32_MODE_USR,
-		.fn		= a32_setend_handler,
-	},
-	{
-		/* Thumb mode */
-		.instr_mask	= 0xfffffff7,
-		.instr_val	= 0x0000b650,
-		.pstate_mask	= (PSR_AA32_T_BIT | PSR_AA32_MODE_MASK),
-		.pstate_val	= (PSR_AA32_T_BIT | PSR_AA32_MODE_USR),
-		.fn		= t16_setend_handler,
-	},
-	{}
-};
+static bool try_emulate_setend(struct pt_regs *regs, u32 insn)
+{
+	if (compat_thumb_mode(regs) &&
+	    (insn & 0xfffffff7) == 0x0000b650)
+		return t16_setend_handler(regs, insn) == 0;
+
+	if (compat_user_mode(regs) &&
+	    (insn & 0xfffffdff) == 0xf1010000)
+		return a32_setend_handler(regs, insn) == 0;
+
+	return false;
+}
 
 static struct insn_emulation insn_setend = {
 	.name = "setend",
 	.status = INSN_DEPRECATED,
-	.hooks = setend_hooks,
+	.try_emulate = try_emulate_setend,
 	.set_hw_mode = setend_set_hw_mode,
 };
+#endif /* CONFIG_SETEND_EMULATION */
 
-static LIST_HEAD(insn_emulation);
-static int nr_insn_emulated __initdata;
-static DEFINE_RAW_SPINLOCK(insn_emulation_lock);
+static struct insn_emulation *insn_emulations[] = {
+#ifdef CONFIG_SWP_EMULATION
+	&insn_swp,
+#endif
+#ifdef CONFIG_CP15_BARRIER_EMULATION
+	&insn_cp15_barrier,
+#endif
+#ifdef CONFIG_SETEND_EMULATION
+	&insn_setend,
+#endif
+};
+
 static DEFINE_MUTEX(insn_emulation_mutex);
-
-static void register_emulation_hooks(struct insn_emulation *insn)
-{
-	struct undef_hook *hook;
-
-	BUG_ON(!insn->hooks);
-
-	for (hook = insn->hooks; hook->instr_mask; hook++)
-		register_undef_hook(hook);
-
-	pr_notice("Registered %s emulation handler\n", insn->name);
-}
-
-static void remove_emulation_hooks(struct insn_emulation *insn)
-{
-	struct undef_hook *hook;
-
-	BUG_ON(!insn->hooks);
-
-	for (hook = insn->hooks; hook->instr_mask; hook++)
-		unregister_undef_hook(hook);
-
-	pr_notice("Removed %s emulation handler\n", insn->name);
-}
 
 static void enable_insn_hw_mode(void *data)
 {
···
 {
 	int rc = 0;
 	unsigned long flags;
-	struct insn_emulation *insn;
 
-	raw_spin_lock_irqsave(&insn_emulation_lock, flags);
-	list_for_each_entry(insn, &insn_emulation, node) {
-		bool enable = (insn->current_mode == INSN_HW);
+	/*
+	 * Disable IRQs to serialize against an IPI from
+	 * run_all_cpu_set_hw_mode(), ensuring the HW is programmed to the most
+	 * recent enablement state if the two race with one another.
+	 */
+	local_irq_save(flags);
+	for (int i = 0; i < ARRAY_SIZE(insn_emulations); i++) {
+		struct insn_emulation *insn = insn_emulations[i];
+		bool enable = READ_ONCE(insn->current_mode) == INSN_HW;
 		if (insn->set_hw_mode && insn->set_hw_mode(enable)) {
 			pr_warn("CPU[%u] cannot support the emulation of %s",
 				cpu, insn->name);
 			rc = -EINVAL;
 		}
 	}
-	raw_spin_unlock_irqrestore(&insn_emulation_lock, flags);
+	local_irq_restore(flags);
+
 	return rc;
 }
 
···
 	case INSN_UNDEF: /* Nothing to be done */
 		break;
 	case INSN_EMULATE:
-		remove_emulation_hooks(insn);
 		break;
 	case INSN_HW:
 		if (!run_all_cpu_set_hw_mode(insn, false))
···
 	case INSN_UNDEF:
 		break;
 	case INSN_EMULATE:
-		register_emulation_hooks(insn);
 		break;
 	case INSN_HW:
 		ret = run_all_cpu_set_hw_mode(insn, true);
···
 	}
 
 	return ret;
-}
-
-static void __init register_insn_emulation(struct insn_emulation *insn)
-{
-	unsigned long flags;
-
-	insn->min = INSN_UNDEF;
-
-	switch (insn->status) {
-	case INSN_DEPRECATED:
-		insn->current_mode = INSN_EMULATE;
-		/* Disable the HW mode if it was turned on at early boot time */
-		run_all_cpu_set_hw_mode(insn, false);
-		insn->max = INSN_HW;
-		break;
-	case INSN_OBSOLETE:
-		insn->current_mode = INSN_UNDEF;
-		insn->max = INSN_EMULATE;
-		break;
-	}
-
-	raw_spin_lock_irqsave(&insn_emulation_lock, flags);
-	list_add(&insn->node, &insn_emulation);
-	nr_insn_emulated++;
-	raw_spin_unlock_irqrestore(&insn_emulation_lock, flags);
-
-	/* Register any handlers if required */
-	update_insn_emulation_mode(insn, INSN_UNDEF);
 }
 
 static int emulation_proc_handler(struct ctl_table *table, int write,
···
 	ret = update_insn_emulation_mode(insn, prev_mode);
 	if (ret) {
 		/* Mode change failed, revert to previous mode. */
-		insn->current_mode = prev_mode;
+		WRITE_ONCE(insn->current_mode, prev_mode);
 		update_insn_emulation_mode(insn, INSN_UNDEF);
 	}
 ret:
···
 	return ret;
 }
 
-static void __init register_insn_emulation_sysctl(void)
+static void __init register_insn_emulation(struct insn_emulation *insn)
 {
-	unsigned long flags;
-	int i = 0;
-	struct insn_emulation *insn;
-	struct ctl_table *insns_sysctl, *sysctl;
+	struct ctl_table *sysctl;
 
-	insns_sysctl = kcalloc(nr_insn_emulated + 1, sizeof(*sysctl),
-			       GFP_KERNEL);
-	if (!insns_sysctl)
-		return;
+	insn->min = INSN_UNDEF;
 
-	raw_spin_lock_irqsave(&insn_emulation_lock, flags);
-	list_for_each_entry(insn, &insn_emulation, node) {
-		sysctl = &insns_sysctl[i];
+	switch (insn->status) {
+	case INSN_DEPRECATED:
+		insn->current_mode = INSN_EMULATE;
+		/* Disable the HW mode if it was turned on at early boot time */
+		run_all_cpu_set_hw_mode(insn, false);
+		insn->max = INSN_HW;
+		break;
+	case INSN_OBSOLETE:
+		insn->current_mode = INSN_UNDEF;
+		insn->max = INSN_EMULATE;
+		break;
+	case INSN_UNAVAILABLE:
+		insn->current_mode = INSN_UNDEF;
+		insn->max = INSN_UNDEF;
+		break;
+	}
+
+	/* Program the HW if required */
+	update_insn_emulation_mode(insn, INSN_UNDEF);
+
+	if (insn->status != INSN_UNAVAILABLE) {
+		sysctl = &insn->sysctl[0];
 
 		sysctl->mode = 0644;
 		sysctl->maxlen = sizeof(int);
···
 		sysctl->extra1 = &insn->min;
 		sysctl->extra2 = &insn->max;
 		sysctl->proc_handler = emulation_proc_handler;
-		i++;
-	}
-	raw_spin_unlock_irqrestore(&insn_emulation_lock, flags);
 
-	register_sysctl("abi", insns_sysctl);
+		register_sysctl("abi", sysctl);
+	}
+}
+
+bool try_emulate_armv8_deprecated(struct pt_regs *regs, u32 insn)
+{
+	for (int i = 0; i < ARRAY_SIZE(insn_emulations); i++) {
+		struct insn_emulation *ie = insn_emulations[i];
+
+		if (ie->status == INSN_UNAVAILABLE)
+			continue;
+
+		/*
+		 * A trap may race with the mode being changed
+		 * INSN_EMULATE<->INSN_HW. Try to emulate the instruction to
+		 * avoid a spurious UNDEF.
+		 */
+		if (READ_ONCE(ie->current_mode) == INSN_UNDEF)
+			continue;
+
+		if (ie->try_emulate(regs, insn))
+			return true;
+	}
+
+	return false;
 }
 
 /*
···
  */
 static int __init armv8_deprecated_init(void)
 {
-	if (IS_ENABLED(CONFIG_SWP_EMULATION))
-		register_insn_emulation(&insn_swp);
+#ifdef CONFIG_SETEND_EMULATION
+	if (!system_supports_mixed_endian_el0()) {
+		insn_setend.status = INSN_UNAVAILABLE;
+		pr_info("setend instruction emulation is not supported on this system\n");
+	}
 
-	if (IS_ENABLED(CONFIG_CP15_BARRIER_EMULATION))
-		register_insn_emulation(&insn_cp15_barrier);
+#endif
+	for (int i = 0; i < ARRAY_SIZE(insn_emulations); i++) {
+		struct insn_emulation *ie = insn_emulations[i];
 
-	if (IS_ENABLED(CONFIG_SETEND_EMULATION)) {
-		if (system_supports_mixed_endian_el0())
-			register_insn_emulation(&insn_setend);
-		else
-			pr_info("setend instruction emulation is not supported on this system\n");
+		if (ie->status == INSN_UNAVAILABLE)
+			continue;
+
+		register_insn_emulation(ie);
 	}
 
 	cpuhp_setup_state_nocalls(CPUHP_AP_ARM64_ISNDEP_STARTING,
 				  "arm64/isndep:starting",
 				  run_all_insn_set_hw_mode, NULL);
-	register_insn_emulation_sysctl();
-
 	return 0;
 }
 

arch/arm64/kernel/traps.c

···
 		regs->pstate &= ~PSR_BTYPE_MASK;
 }
 
-static LIST_HEAD(undef_hook);
-static DEFINE_RAW_SPINLOCK(undef_lock);
-
-void register_undef_hook(struct undef_hook *hook)
-{
-	unsigned long flags;
-
-	raw_spin_lock_irqsave(&undef_lock, flags);
-	list_add(&hook->node, &undef_hook);
-	raw_spin_unlock_irqrestore(&undef_lock, flags);
-}
-
-void unregister_undef_hook(struct undef_hook *hook)
-{
-	unsigned long flags;
-
-	raw_spin_lock_irqsave(&undef_lock, flags);
-	list_del(&hook->node);
-	raw_spin_unlock_irqrestore(&undef_lock, flags);
-}
-
 static int user_insn_read(struct pt_regs *regs, u32 *insnp)
 {
 	u32 instr;
···
 
 	*insnp = instr;
 	return 0;
-}
-
-static int call_undef_hook(struct pt_regs *regs, u32 instr)
-{
-	struct undef_hook *hook;
-	unsigned long flags;
-	int (*fn)(struct pt_regs *regs, u32 instr) = NULL;
-
-	raw_spin_lock_irqsave(&undef_lock, flags);
-	list_for_each_entry(hook, &undef_hook, node)
-		if ((instr & hook->instr_mask) == hook->instr_val &&
-			(regs->pstate & hook->pstate_mask) == hook->pstate_val)
-			fn = hook->fn;
-
-	raw_spin_unlock_irqrestore(&undef_lock, flags);
-
-	return fn ? fn(regs, instr) : 1;
 }
 
 void force_signal_inject(int signal, int code, unsigned long address, unsigned long err)
···
 	if (try_emulate_mrs(regs, insn))
 		return;
 
-	if (call_undef_hook(regs, insn) == 0)
+	if (try_emulate_armv8_deprecated(regs, insn))
 		return;
 
 out_err: