为KGDB 增加watchpoint断点支持 on x86

A：using gdb to set a watchpoint 
 
B：send out a set watchpoint protocol packet to kgdb from gdb
 
C：kgdb receive/parse the protocol packet
 
D：kgdb set a watchpoint hardware breakpoint on kernel
 
E：Once kernel hit a watchpoint breakpoint, kgdb will collect the 
    watchpoint breakpoint info, fill them to a Stop-Reply-Packets with 
    watchpoint format, and send out to gdb

 (gdb) watch xxx  (假设我们要监视断点地址为 xxx)
 
 gdb发送数据包"Z2,xxx,4" 给kgdb
 
 kgdb 收到协议数据包后，调用gdb_cmd_break()(kernel/debug/gdbstub.c)
 来解析设置断点协议数据
 
gdb_cmd_break()
------------------------------------------
parsing  "Z2,xxx,4"
--> 'Z'      设置断点
--> '2'      断点类型为write watchpoint
--> 'xxx'  断点地址为 xxx
--> '4'      断点长度为 4
------------------------------------------
 
--> arch_kgdb_ops.set_hw_breakpoint(xxx, 4, BP_WRITE_WATCHPOINT)
--> kgdb_set_hw_break(xxx, 4, BP_WRITE_WATCHPOINT) (arch/x86/kernel/kgdb.c)
    这个函数完成了将断点信息加入到breakinfo[HBP_NUM]数组里和设置其enabled标志为1，
    真正的断点使能是在kgdb退出，让系统正常运行时，调用kgdb_correct_hw_break()函数
    来每个cpu上使能硬件断点.

硬件断点的触发后，会产生一个调试异常,因此会进入do_debug异常的
处理程序(arch/x86/kernel/traps.c).
 
do_debug() {
 
unsigned long dr6;
get_debugreg(dr6, 6); //保持dr6寄存器值到dr6变量
 
 /* DR6 may or may not be cleared by the CPU */
set_debugreg(0, 6); //手动清除dr6寄存器
 
//通过notify_die通知链告诉大家"debug"异常发生了，同时把dr6寄存器值作为参数传递出去。
notify_die(DIE_DEBUG, "debug", regs, PTR_ERR(&dr6), error_code,
                            SIGTRAP)
}

Hardware Breakpoint Layer处理流程:
 
static struct notifier_block hw_breakpoint_exceptions_nb = {
    .notifier_call = hw_breakpoint_exceptions_notify,
    /* we need to be notified first */
    .priority = 0x7fffffff
};
register_die_notifier(&hw_breakpoint_exceptions_nb);
 
触发硬件断点:
do_debug() -- notify_die(DIE_DEBUG)->
arch/x86/kernel/hw_breakpoint.c
hw_breakpoint_exceptions_notify() -> 
hw_breakpoint_handler() {
for (i = 0; i < HBP_NUM; ++i) {
        if (likely(!(dr6 & (DR_TRAP0 << i))))
            continue;
        perf_bp_event(bp, args->regs); -->
           perf_swevent_overflow(event)->
             event->overflow_handler(event, nmi, data, regs);-->
                kgdb_hw_overflow_handler(). 这里最终跑到kgdb的代码里了.
}/* end of for()*/
}

static void kgdb_hw_overflow_handler(struct perf_event *event, int nmi,
        struct perf_sample_data *data, struct pt_regs *regs) {
 for (i = 0; i < HBP_NUM; i++) {
        if (!breakinfo[i].enabled)
            continue;
        if (breakinfo[i].addr == event->attr.bp_addr) //查找和判断踩中断点信息
            break;
    }
}

type = breakinfo[i].type;
addr = breakinfo[i].addr;
            switch (type) {
            case KST_READ_WATCHPOINT:
                ptr += strlen(strcpy(ptr, "rwatch:"));
                break;
            case KST_ACCESS_WATCHPOINT:
                ptr += strlen(strcpy(ptr, "awatch:"));
                break;
            case KST_WRITE_WATCHPOINT:
                ptr += strlen(strcpy(ptr, "watch:"));
                break;
            }
            ptr += kgdb_long2hex(addr, ptr);
            *ptr++ = ';';

After gdb connect successly to kgdb, setting a watchpoint
at softlockup_thresh address.
 
(gdb) watch softlockup_thresh
 
Then change the watchdog_thresh value to 24 on target:
# echo "24" > /proc/sys/kernel/watchdog_thresh
 
Then the gdb will stop and show:
----------------------------------------------------------
Old value = 60
New value = 24
0xffffffff810481f2 in do_proc_dointvec_minmax_conv (negp=0xffff88066cbb9e17,
lvalp=0xffffffff8190e718, 
    valp=0xffffffff8190e718, write=<value optimized out>,
data=0xffff88066cbb9e68) at kernel/sysctl.c:2408
2408			*valp = val;
(gdb)
----------------------------------------------------------
 
Change the watchdog_thresh value to 24 on target again:
# echo "24" > /proc/sys/kernel/watchdog_thresh
 
As the previous watchdog_thresh value is 24, thus gdb will not stop..