GDB Remote debug: Why several "$Z0,address,length" msg when only set one breakpoint?

[Luis Machado]

On 08/26/2014 03:57 AM, Peng Fan wrote:
> Hi,
>
> Currently, I am hacking uboot arm kgdb and adding breakpoint support.
>
> The steps is as following:
> (gdb) b do_bootz
> Breakpoint 1, do_bootz (cmdtp=0x8784ca14 <_u_boot_list_2_cmd_2_bootz>, flag=0, argc=1, argv=0xbf552a20) at common/cmd_bootm.c:1928
> (gdb) c
> Continuing.
> Sending packet: $m87809580,4#7a...Ack
> Packet received: f0452de9
> Sending packet: $m87809580,4#7a...Ack
> Packet received: f0452de9
> Sending packet: $Z0,87809584,4#c7...Ack
> Packet received: OK
> Sending packet: $Hc0#db...Ack
> Packet received:
> Sending packet: $c#63...Ack
> Packet received: T050f:84958087;0d:a41355bf;
> Sending packet: $z0,87809584,4#e7...Ack
> Packet received: OK
> Sending packet: $qTStatus#49...Ack
> Packet received:
> Sending packet: $Z0,87800000,4#ad...Ack
> Packet received: OK
> Sending packet: $Z0,87809580,4#c3...Ack
> Packet received: OK
> Sending packet: $Hc0#db...Ack
> Packet received:
> Sending packet: $c#63...Ack
>
> Actually the breakpoint is at 0x87809580, but why first set breakpoint at 0x87809584 then remove it then set breakpoint at 0x87809580?
>
> Also the first time set a breakpoint then continue.
> (gdb) c
> Continuing.
> Sending packet: $qTStatus#49...Ack
> Packet received:
> Sending packet: $Z0,87800000,4#ad...Ack
> Packet received: OK
> Packet Z0 (software-breakpoint) is supported
> Sending packet: $Z0,87809580,4#c3...Ack
> Packet received: OK
> Sending packet: $vCont?#49...Ack
> Packet received:
> Packet vCont (verbose-resume) is NOT supported
> Sending packet: $Hc0#db...Ack
> Packet received:
> Sending packet: $c#63...Ack
>
> why two Z0 here? I only set break at 0x87809580, but gdb send a break set at 0x87800000 which is the beginning of the elf entry.
>
> Thanks.
> Peng.
>
>

ARM does not have hardware single-stepping support, so GDB needs to 
simulate it with continue requests and breakpoints.

Regarding additional breakpoints, GDB has its own internal breakpoints, 
so your session may not be restricted to the breakpoints you created.

On third reason, depending on the mode, GDB inserts and removes 
breakpoints with every program stop.