arch/arm64/kernel/debug-monitors.c - arm/linux-arm-legacy - Git at Google

 /*
  * ARMv8 single-step debug support and mdscr context switching.
  *
  * Copyright (C) 2012 ARM Limited
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 as
  * published by the Free Software Foundation.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  * Author: Will Deacon <will.deacon@arm.com>
  */

 #include <linux/cpu.h>
 #include <linux/debugfs.h>
 #include <linux/hardirq.h>
 #include <linux/init.h>
 #include <linux/ptrace.h>
 #include <linux/stat.h>
 #include <linux/uaccess.h>

 #include <asm/debug-monitors.h>
 #include <asm/cputype.h>
 #include <asm/system_misc.h>

 /* Low-level stepping controls. */
 #define DBG_MDSCR_SS		(1 << 0)
 #define DBG_SPSR_SS		(1 << 21)

 /* MDSCR_EL1 enabling bits */
 #define DBG_MDSCR_KDE		(1 << 13)
 #define DBG_MDSCR_MDE		(1 << 15)
 #define DBG_MDSCR_MASK		~(DBG_MDSCR_KDE | DBG_MDSCR_MDE)

 /* Determine debug architecture. */
 u8 debug_monitors_arch(void)
 {
 	return read_cpuid(ID_AA64DFR0_EL1) & 0xf;
 }

 /*
  * MDSCR access routines.
  */
 static void mdscr_write(u32 mdscr)
 {
 	unsigned long flags;
 	local_dbg_save(flags);
 	asm volatile("msr mdscr_el1, %0" :: "r" ((u64)mdscr));
 	local_dbg_restore(flags);
 }

 static u32 mdscr_read(void)
 {
 	u64 mdscr;
 	asm volatile("mrs %0, mdscr_el1" : "=r" (mdscr));
 	return (u32)mdscr;
 }

 /*
  * Allow root to disable self-hosted debug from userspace.
  * This is useful if you want to connect an external JTAG debugger.
  */
 static u32 debug_enabled = 1;

 static int create_debug_debugfs_entry(void)
 {
 	debugfs_create_bool("debug_enabled", 0644, NULL, &debug_enabled);
 	return 0;
 }
 fs_initcall(create_debug_debugfs_entry);

 static int __init early_debug_disable(char *buf)
 {
 	debug_enabled = 0;
 	return 0;
 }

 early_param("nodebugmon", early_debug_disable);

 /*
  * Keep track of debug users on each core.
  * The ref counts are per-cpu so we use a local_t type.
  */
 static DEFINE_PER_CPU(int, mde_ref_count);
 static DEFINE_PER_CPU(int, kde_ref_count);

 void enable_debug_monitors(enum debug_el el)
 {
 	u32 mdscr, enable = 0;

 	WARN_ON(preemptible());

 	if (this_cpu_inc_return(mde_ref_count) == 1)
 		enable = DBG_MDSCR_MDE;

 	if (el == DBG_ACTIVE_EL1 &&
 	    this_cpu_inc_return(kde_ref_count) == 1)
 		enable |= DBG_MDSCR_KDE;

 	if (enable && debug_enabled) {
 		mdscr = mdscr_read();
 		mdscr |= enable;
 		mdscr_write(mdscr);
 	}
 }

 void disable_debug_monitors(enum debug_el el)
 {
 	u32 mdscr, disable = 0;

 	WARN_ON(preemptible());

 	if (this_cpu_dec_return(mde_ref_count) == 0)
 		disable = ~DBG_MDSCR_MDE;

 	if (el == DBG_ACTIVE_EL1 &&
 	    this_cpu_dec_return(kde_ref_count) == 0)
 		disable &= ~DBG_MDSCR_KDE;

 	if (disable) {
 		mdscr = mdscr_read();
 		mdscr &= disable;
 		mdscr_write(mdscr);
 	}
 }

 /*
  * OS lock clearing.
  */
 static void clear_os_lock(void *unused)
 {
 	asm volatile("msr oslar_el1, %0" : : "r" ((u64)0));
 }

 static int os_lock_notify(struct notifier_block *self,
 				    unsigned long action, void *data)
 {
 	int cpu = (unsigned long)data;
 	if (action == CPU_ONLINE)
 		smp_call_function_single(cpu, clear_os_lock, NULL, 1);
 	return NOTIFY_OK;
 }

 static struct notifier_block os_lock_nb = {
 	.notifier_call = os_lock_notify,
 };

 static int debug_monitors_init(void)
 {
 	/* Clear the OS lock. */
 	on_each_cpu(clear_os_lock, NULL, 1);
 	isb();
 	local_dbg_enable();

 	/* Register hotplug handler. */
 	register_cpu_notifier(&os_lock_nb);
 	return 0;
 }
 postcore_initcall(debug_monitors_init);

 /*
  * Single step API and exception handling.
  */
 static void set_regs_spsr_ss(struct pt_regs *regs)
 {
 	unsigned long spsr;

 	spsr = regs->pstate;
 	spsr &= ~DBG_SPSR_SS;
 	spsr |= DBG_SPSR_SS;
 	regs->pstate = spsr;
 }

 static void clear_regs_spsr_ss(struct pt_regs *regs)
 {
 	unsigned long spsr;

 	spsr = regs->pstate;
 	spsr &= ~DBG_SPSR_SS;
 	regs->pstate = spsr;
 }

 /* EL1 Single Step Handler hooks */
 static LIST_HEAD(step_hook);
 static DEFINE_RWLOCK(step_hook_lock);

 void register_step_hook(struct step_hook *hook)
 {
 	write_lock(&step_hook_lock);
 	list_add(&hook->node, &step_hook);
 	write_unlock(&step_hook_lock);
 }

 void unregister_step_hook(struct step_hook *hook)
 {
 	write_lock(&step_hook_lock);
 	list_del(&hook->node);
 	write_unlock(&step_hook_lock);
 }

 /*
  * Call registered single step handers
  * There is no Syndrome info to check for determining the handler.
  * So we call all the registered handlers, until the right handler is
  * found which returns zero.
  */
 static int call_step_hook(struct pt_regs *regs, unsigned int esr)
 {
 	struct step_hook *hook;
 	int retval = DBG_HOOK_ERROR;

 	read_lock(&step_hook_lock);

 	list_for_each_entry(hook, &step_hook, node)	{
 		retval = hook->fn(regs, esr);
 		if (retval == DBG_HOOK_HANDLED)
 			break;
 	}

 	read_unlock(&step_hook_lock);

 	return retval;
 }

 static int single_step_handler(unsigned long addr, unsigned int esr,
 			       struct pt_regs *regs)
 {
 	siginfo_t info;

 	/*
 	 * If we are stepping a pending breakpoint, call the hw_breakpoint
 	 * handler first.
 	 */
 	if (!reinstall_suspended_bps(regs))
 		return 0;

 	if (user_mode(regs)) {
 		info.si_signo = SIGTRAP;
 		info.si_errno = 0;
 		info.si_code  = TRAP_HWBKPT;
 		info.si_addr  = (void __user *)instruction_pointer(regs);
 		force_sig_info(SIGTRAP, &info, current);

 		/*
 		 * ptrace will disable single step unless explicitly
 		 * asked to re-enable it. For other clients, it makes
 		 * sense to leave it enabled (i.e. rewind the controls
 		 * to the active-not-pending state).
 		 */
 		user_rewind_single_step(current);
 	} else {
 		if (call_step_hook(regs, esr) == DBG_HOOK_HANDLED)
 			return 0;

 		pr_warning("Unexpected kernel single-step exception at EL1\n");
 		/*
 		 * Re-enable stepping since we know that we will be
 		 * returning to regs.
 		 */
 		set_regs_spsr_ss(regs);
 	}

 	return 0;
 }

 /*
  * Breakpoint handler is re-entrant as another breakpoint can
  * hit within breakpoint handler, especically in kprobes.
  * Use reader/writer locks instead of plain spinlock.
  */
 static LIST_HEAD(break_hook);
 static DEFINE_RWLOCK(break_hook_lock);

 void register_break_hook(struct break_hook *hook)
 {
 	write_lock(&break_hook_lock);
 	list_add(&hook->node, &break_hook);
 	write_unlock(&break_hook_lock);
 }

 void unregister_break_hook(struct break_hook *hook)
 {
 	write_lock(&break_hook_lock);
 	list_del(&hook->node);
 	write_unlock(&break_hook_lock);
 }

 static int call_break_hook(struct pt_regs *regs, unsigned int esr)
 {
 	struct break_hook *hook;
 	int (*fn)(struct pt_regs *regs, unsigned int esr) = NULL;

 	read_lock(&break_hook_lock);
 	list_for_each_entry(hook, &break_hook, node)
 		if ((esr & hook->esr_mask) == hook->esr_val)
 			fn = hook->fn;
 	read_unlock(&break_hook_lock);

 	return fn ? fn(regs, esr) : DBG_HOOK_ERROR;
 }

 static int brk_handler(unsigned long addr, unsigned int esr,
 		       struct pt_regs *regs)
 {
 	siginfo_t info;

 	if (call_break_hook(regs, esr) == DBG_HOOK_HANDLED)
 		return 0;

 	pr_warn("unexpected brk exception at %lx, esr=0x%x\n",
 			(long)instruction_pointer(regs), esr);

 	if (!user_mode(regs))
 		return -EFAULT;

 	info = (siginfo_t) {
 		.si_signo = SIGTRAP,
 		.si_errno = 0,
 		.si_code  = TRAP_BRKPT,
 		.si_addr  = (void __user *)instruction_pointer(regs),
 	};

 	force_sig_info(SIGTRAP, &info, current);
 	return 0;
 }

 int aarch32_break_handler(struct pt_regs *regs)
 {
 	siginfo_t info;
 	u32 arm_instr;
 	u16 thumb_instr;
 	bool bp = false;
 	void __user *pc = (void __user *)instruction_pointer(regs);

 	if (!compat_user_mode(regs))
 		return -EFAULT;

 	if (compat_thumb_mode(regs)) {
 		/* get 16-bit Thumb instruction */
 		get_user(thumb_instr, (u16 __user *)pc);
 		thumb_instr = le16_to_cpu(thumb_instr);
 		if (thumb_instr == AARCH32_BREAK_THUMB2_LO) {
 			/* get second half of 32-bit Thumb-2 instruction */
 			get_user(thumb_instr, (u16 __user *)(pc + 2));
 			thumb_instr = le16_to_cpu(thumb_instr);
 			bp = thumb_instr == AARCH32_BREAK_THUMB2_HI;
 		} else {
 			bp = thumb_instr == AARCH32_BREAK_THUMB;
 		}
 	} else {
 		/* 32-bit ARM instruction */
 		get_user(arm_instr, (u32 __user *)pc);
 		arm_instr = le32_to_cpu(arm_instr);
 		bp = (arm_instr & ~0xf0000000) == AARCH32_BREAK_ARM;
 	}

 	if (!bp)
 		return -EFAULT;

 	info = (siginfo_t) {
 		.si_signo = SIGTRAP,
 		.si_errno = 0,
 		.si_code  = TRAP_BRKPT,
 		.si_addr  = pc,
 	};

 	force_sig_info(SIGTRAP, &info, current);
 	return 0;
 }

 static int __init debug_traps_init(void)
 {
 	hook_debug_fault_code(DBG_ESR_EVT_HWSS, single_step_handler, SIGTRAP,
 			      TRAP_HWBKPT, "single-step handler");
 	hook_debug_fault_code(DBG_ESR_EVT_BRK, brk_handler, SIGTRAP,
 			      TRAP_BRKPT, "ptrace BRK handler");
 	return 0;
 }
 arch_initcall(debug_traps_init);

 /* Re-enable single step for syscall restarting. */
 void user_rewind_single_step(struct task_struct *task)
 {
 	/*
 	 * If single step is active for this thread, then set SPSR.SS
 	 * to 1 to avoid returning to the active-pending state.
 	 */
 	if (test_ti_thread_flag(task_thread_info(task), TIF_SINGLESTEP))
 		set_regs_spsr_ss(task_pt_regs(task));
 }

 void user_fastforward_single_step(struct task_struct *task)
 {
 	if (test_ti_thread_flag(task_thread_info(task), TIF_SINGLESTEP))
 		clear_regs_spsr_ss(task_pt_regs(task));
 }

 /* Kernel API */
 void kernel_enable_single_step(struct pt_regs *regs)
 {
 	WARN_ON(!irqs_disabled());
 	set_regs_spsr_ss(regs);
 	mdscr_write(mdscr_read() | DBG_MDSCR_SS);
 	enable_debug_monitors(DBG_ACTIVE_EL1);
 }

 void kernel_disable_single_step(void)
 {
 	WARN_ON(!irqs_disabled());
 	mdscr_write(mdscr_read() & ~DBG_MDSCR_SS);
 	disable_debug_monitors(DBG_ACTIVE_EL1);
 }

 int kernel_active_single_step(void)
 {
 	WARN_ON(!irqs_disabled());
 	return mdscr_read() & DBG_MDSCR_SS;
 }

 /* ptrace API */
 void user_enable_single_step(struct task_struct *task)
 {
 	set_ti_thread_flag(task_thread_info(task), TIF_SINGLESTEP);
 	set_regs_spsr_ss(task_pt_regs(task));
 }

 void user_disable_single_step(struct task_struct *task)
 {
 	clear_ti_thread_flag(task_thread_info(task), TIF_SINGLESTEP);
 }
	/*
	* ARMv8 single-step debug support and mdscr context switching.
	*
	* Copyright (C) 2012 ARM Limited
	*
	* This program is free software; you can redistribute it and/or modify
	* it under the terms of the GNU General Public License version 2 as
	* published by the Free Software Foundation.
	*
	* This program is distributed in the hope that it will be useful,
	* but WITHOUT ANY WARRANTY; without even the implied warranty of
	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	* GNU General Public License for more details.
	*
	* You should have received a copy of the GNU General Public License
	* along with this program. If not, see <http://www.gnu.org/licenses/>.
	*
	* Author: Will Deacon <will.deacon@arm.com>
	*/

	#include <linux/cpu.h>
	#include <linux/debugfs.h>
	#include <linux/hardirq.h>
	#include <linux/init.h>
	#include <linux/ptrace.h>
	#include <linux/stat.h>
	#include <linux/uaccess.h>

	#include <asm/debug-monitors.h>
	#include <asm/cputype.h>
	#include <asm/system_misc.h>

	/* Low-level stepping controls. */
	#define DBG_MDSCR_SS (1 << 0)
	#define DBG_SPSR_SS (1 << 21)

	/* MDSCR_EL1 enabling bits */
	#define DBG_MDSCR_KDE (1 << 13)
	#define DBG_MDSCR_MDE (1 << 15)
	#define DBG_MDSCR_MASK ~(DBG_MDSCR_KDE \| DBG_MDSCR_MDE)

	/* Determine debug architecture. */
	u8 debug_monitors_arch(void)
	{
	return read_cpuid(ID_AA64DFR0_EL1) & 0xf;
	}

	/*
	* MDSCR access routines.
	*/
	static void mdscr_write(u32 mdscr)
	{
	unsigned long flags;
	local_dbg_save(flags);
	asm volatile("msr mdscr_el1, %0" :: "r" ((u64)mdscr));
	local_dbg_restore(flags);
	}

	static u32 mdscr_read(void)
	{
	u64 mdscr;
	asm volatile("mrs %0, mdscr_el1" : "=r" (mdscr));
	return (u32)mdscr;
	}

	/*
	* Allow root to disable self-hosted debug from userspace.
	* This is useful if you want to connect an external JTAG debugger.
	*/
	static u32 debug_enabled = 1;

	static int create_debug_debugfs_entry(void)
	{
	debugfs_create_bool("debug_enabled", 0644, NULL, &debug_enabled);
	return 0;
	}
	fs_initcall(create_debug_debugfs_entry);

	static int __init early_debug_disable(char *buf)
	{
	debug_enabled = 0;
	return 0;
	}

	early_param("nodebugmon", early_debug_disable);

	/*
	* Keep track of debug users on each core.
	* The ref counts are per-cpu so we use a local_t type.
	*/
	static DEFINE_PER_CPU(int, mde_ref_count);
	static DEFINE_PER_CPU(int, kde_ref_count);

	void enable_debug_monitors(enum debug_el el)
	{
	u32 mdscr, enable = 0;

	WARN_ON(preemptible());

	if (this_cpu_inc_return(mde_ref_count) == 1)
	enable = DBG_MDSCR_MDE;

	if (el == DBG_ACTIVE_EL1 &&
	this_cpu_inc_return(kde_ref_count) == 1)
	enable \|= DBG_MDSCR_KDE;

	if (enable && debug_enabled) {
	mdscr = mdscr_read();
	mdscr \|= enable;
	mdscr_write(mdscr);
	}
	}

	void disable_debug_monitors(enum debug_el el)
	{
	u32 mdscr, disable = 0;

	WARN_ON(preemptible());

	if (this_cpu_dec_return(mde_ref_count) == 0)
	disable = ~DBG_MDSCR_MDE;

	if (el == DBG_ACTIVE_EL1 &&
	this_cpu_dec_return(kde_ref_count) == 0)
	disable &= ~DBG_MDSCR_KDE;

	if (disable) {
	mdscr = mdscr_read();
	mdscr &= disable;
	mdscr_write(mdscr);
	}
	}

	/*
	* OS lock clearing.
	*/
	static void clear_os_lock(void *unused)
	{
	asm volatile("msr oslar_el1, %0" : : "r" ((u64)0));
	}

	static int os_lock_notify(struct notifier_block *self,
	unsigned long action, void *data)
	{
	int cpu = (unsigned long)data;
	if (action == CPU_ONLINE)
	smp_call_function_single(cpu, clear_os_lock, NULL, 1);
	return NOTIFY_OK;
	}

	static struct notifier_block os_lock_nb = {
	.notifier_call = os_lock_notify,
	};

	static int debug_monitors_init(void)
	{
	/* Clear the OS lock. */
	on_each_cpu(clear_os_lock, NULL, 1);
	isb();
	local_dbg_enable();

	/* Register hotplug handler. */
	register_cpu_notifier(&os_lock_nb);
	return 0;
	}
	postcore_initcall(debug_monitors_init);

	/*
	* Single step API and exception handling.
	*/
	static void set_regs_spsr_ss(struct pt_regs *regs)
	{
	unsigned long spsr;

	spsr = regs->pstate;
	spsr &= ~DBG_SPSR_SS;
	spsr \|= DBG_SPSR_SS;
	regs->pstate = spsr;
	}

	static void clear_regs_spsr_ss(struct pt_regs *regs)
	{
	unsigned long spsr;

	spsr = regs->pstate;
	spsr &= ~DBG_SPSR_SS;
	regs->pstate = spsr;
	}

	/* EL1 Single Step Handler hooks */
	static LIST_HEAD(step_hook);
	static DEFINE_RWLOCK(step_hook_lock);

	void register_step_hook(struct step_hook *hook)
	{
	write_lock(&step_hook_lock);
	list_add(&hook->node, &step_hook);
	write_unlock(&step_hook_lock);
	}

	void unregister_step_hook(struct step_hook *hook)
	{
	write_lock(&step_hook_lock);
	list_del(&hook->node);
	write_unlock(&step_hook_lock);
	}

	/*
	* Call registered single step handers
	* There is no Syndrome info to check for determining the handler.
	* So we call all the registered handlers, until the right handler is
	* found which returns zero.
	*/
	static int call_step_hook(struct pt_regs *regs, unsigned int esr)
	{
	struct step_hook *hook;
	int retval = DBG_HOOK_ERROR;

	read_lock(&step_hook_lock);

	list_for_each_entry(hook, &step_hook, node) {
	retval = hook->fn(regs, esr);
	if (retval == DBG_HOOK_HANDLED)
	break;
	}

	read_unlock(&step_hook_lock);

	return retval;
	}

	static int single_step_handler(unsigned long addr, unsigned int esr,
	struct pt_regs *regs)
	{
	siginfo_t info;

	/*
	* If we are stepping a pending breakpoint, call the hw_breakpoint
	* handler first.
	*/
	if (!reinstall_suspended_bps(regs))
	return 0;

	if (user_mode(regs)) {
	info.si_signo = SIGTRAP;
	info.si_errno = 0;
	info.si_code = TRAP_HWBKPT;
	info.si_addr = (void __user *)instruction_pointer(regs);
	force_sig_info(SIGTRAP, &info, current);

	/*
	* ptrace will disable single step unless explicitly
	* asked to re-enable it. For other clients, it makes
	* sense to leave it enabled (i.e. rewind the controls
	* to the active-not-pending state).
	*/
	user_rewind_single_step(current);
	} else {
	if (call_step_hook(regs, esr) == DBG_HOOK_HANDLED)
	return 0;

	pr_warning("Unexpected kernel single-step exception at EL1\n");
	/*
	* Re-enable stepping since we know that we will be
	* returning to regs.
	*/
	set_regs_spsr_ss(regs);
	}

	return 0;
	}

	/*
	* Breakpoint handler is re-entrant as another breakpoint can
	* hit within breakpoint handler, especically in kprobes.
	* Use reader/writer locks instead of plain spinlock.
	*/
	static LIST_HEAD(break_hook);
	static DEFINE_RWLOCK(break_hook_lock);

	void register_break_hook(struct break_hook *hook)
	{
	write_lock(&break_hook_lock);
	list_add(&hook->node, &break_hook);
	write_unlock(&break_hook_lock);
	}

	void unregister_break_hook(struct break_hook *hook)
	{
	write_lock(&break_hook_lock);
	list_del(&hook->node);
	write_unlock(&break_hook_lock);
	}

	static int call_break_hook(struct pt_regs *regs, unsigned int esr)
	{
	struct break_hook *hook;
	int (fn)(struct pt_regs regs, unsigned int esr) = NULL;

	read_lock(&break_hook_lock);
	list_for_each_entry(hook, &break_hook, node)
	if ((esr & hook->esr_mask) == hook->esr_val)
	fn = hook->fn;
	read_unlock(&break_hook_lock);

	return fn ? fn(regs, esr) : DBG_HOOK_ERROR;
	}

	static int brk_handler(unsigned long addr, unsigned int esr,
	struct pt_regs *regs)
	{
	siginfo_t info;

	if (call_break_hook(regs, esr) == DBG_HOOK_HANDLED)
	return 0;

	pr_warn("unexpected brk exception at %lx, esr=0x%x\n",
	(long)instruction_pointer(regs), esr);

	if (!user_mode(regs))
	return -EFAULT;

	info = (siginfo_t) {
	.si_signo = SIGTRAP,
	.si_errno = 0,
	.si_code = TRAP_BRKPT,
	.si_addr = (void __user *)instruction_pointer(regs),
	};

	force_sig_info(SIGTRAP, &info, current);
	return 0;
	}

	int aarch32_break_handler(struct pt_regs *regs)
	{
	siginfo_t info;
	u32 arm_instr;
	u16 thumb_instr;
	bool bp = false;
	void __user pc = (void __user )instruction_pointer(regs);

	if (!compat_user_mode(regs))
	return -EFAULT;

	if (compat_thumb_mode(regs)) {
	/* get 16-bit Thumb instruction */
	get_user(thumb_instr, (u16 __user *)pc);
	thumb_instr = le16_to_cpu(thumb_instr);
	if (thumb_instr == AARCH32_BREAK_THUMB2_LO) {
	/* get second half of 32-bit Thumb-2 instruction */
	get_user(thumb_instr, (u16 __user *)(pc + 2));
	thumb_instr = le16_to_cpu(thumb_instr);
	bp = thumb_instr == AARCH32_BREAK_THUMB2_HI;
	} else {
	bp = thumb_instr == AARCH32_BREAK_THUMB;
	}
	} else {
	/* 32-bit ARM instruction */
	get_user(arm_instr, (u32 __user *)pc);
	arm_instr = le32_to_cpu(arm_instr);
	bp = (arm_instr & ~0xf0000000) == AARCH32_BREAK_ARM;
	}

	if (!bp)
	return -EFAULT;

	info = (siginfo_t) {
	.si_signo = SIGTRAP,
	.si_errno = 0,
	.si_code = TRAP_BRKPT,
	.si_addr = pc,
	};

	force_sig_info(SIGTRAP, &info, current);
	return 0;
	}

	static int __init debug_traps_init(void)
	{
	hook_debug_fault_code(DBG_ESR_EVT_HWSS, single_step_handler, SIGTRAP,
	TRAP_HWBKPT, "single-step handler");
	hook_debug_fault_code(DBG_ESR_EVT_BRK, brk_handler, SIGTRAP,
	TRAP_BRKPT, "ptrace BRK handler");
	return 0;
	}
	arch_initcall(debug_traps_init);

	/* Re-enable single step for syscall restarting. */
	void user_rewind_single_step(struct task_struct *task)
	{
	/*
	* If single step is active for this thread, then set SPSR.SS
	* to 1 to avoid returning to the active-pending state.
	*/
	if (test_ti_thread_flag(task_thread_info(task), TIF_SINGLESTEP))
	set_regs_spsr_ss(task_pt_regs(task));
	}

	void user_fastforward_single_step(struct task_struct *task)
	{
	if (test_ti_thread_flag(task_thread_info(task), TIF_SINGLESTEP))
	clear_regs_spsr_ss(task_pt_regs(task));
	}

	/* Kernel API */
	void kernel_enable_single_step(struct pt_regs *regs)
	{
	WARN_ON(!irqs_disabled());
	set_regs_spsr_ss(regs);
	mdscr_write(mdscr_read() \| DBG_MDSCR_SS);
	enable_debug_monitors(DBG_ACTIVE_EL1);
	}

	void kernel_disable_single_step(void)
	{
	WARN_ON(!irqs_disabled());
	mdscr_write(mdscr_read() & ~DBG_MDSCR_SS);
	disable_debug_monitors(DBG_ACTIVE_EL1);
	}

	int kernel_active_single_step(void)
	{
	WARN_ON(!irqs_disabled());
	return mdscr_read() & DBG_MDSCR_SS;
	}

	/* ptrace API */
	void user_enable_single_step(struct task_struct *task)
	{
	set_ti_thread_flag(task_thread_info(task), TIF_SINGLESTEP);
	set_regs_spsr_ss(task_pt_regs(task));
	}

	void user_disable_single_step(struct task_struct *task)
	{
	clear_ti_thread_flag(task_thread_info(task), TIF_SINGLESTEP);
	}