debug-monitors.c

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/*
 * 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 <[email protected]>
 */

#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 <asm/debug-monitors.h>
#include <asm/local.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" (mdscr));
    local_dbg_restore(flags);
}

static u32 mdscr_read(void)
{
    u32 mdscr;
    asm volatile("mrs %0, mdscr_el1" : "=r" (mdscr));
    return 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(local_t, mde_ref_count);
static DEFINE_PER_CPU(local_t, kde_ref_count);

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

    WARN_ON(preemptible());

    if (local_inc_return(&__get_cpu_var(mde_ref_count)) == 1)
        enable = DBG_MDSCR_MDE;

    if (el == DBG_ACTIVE_EL1 &&
        local_inc_return(&__get_cpu_var(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 (local_dec_and_test(&__get_cpu_var(mde_ref_count)))
        disable = ~DBG_MDSCR_MDE;

    if (el == DBG_ACTIVE_EL1 &&
        local_dec_and_test(&__get_cpu_var(kde_ref_count)))
        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 mdscr_el1, %0" : : "r" (0));
    isb();
    asm volatile("msr oslar_el1, %0" : : "r" (0));
    isb();
}

static int __cpuinit 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 __cpuinitdata os_lock_nb = {
    .notifier_call = os_lock_notify,
};

static int __cpuinit debug_monitors_init(void)
{
    /* Clear the OS lock. */
    smp_call_function(clear_os_lock, NULL, 1);
    clear_os_lock(NULL);

    /* 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;
}

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 {
        /* TODO: route to KGDB */
        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;
}

static int __init single_step_init(void)
{
    hook_debug_fault_code(DBG_ESR_EVT_HWSS, single_step_handler, SIGTRAP,
                  TRAP_HWBKPT, "single-step handler");
    return 0;
}
arch_initcall(single_step_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);
}