Re: Hardware watchpoint for read

[Xin Tong <xerox.time.tech@gmail.com>]

On Tue, Apr 24, 2012 at 10:19 AM, Luis Gustavo <luis_gustavo@mentor.com> wrote:
> On 04/24/2012 11:17 AM, Xin Tong wrote:
>>
>> On Tue, Apr 24, 2012 at 9:18 AM, Luis Gustavo<luis_gustavo@mentor.com>
>>  wrote:
>>>
>>> On 04/24/2012 10:15 AM, Xin Tong wrote:
>>>>
>>>>
>>>> On Tue, Apr 24, 2012 at 9:09 AM, Luis Gustavo<luis_gustavo@mentor.com>
>>>>  wrote:
>>>>>
>>>>>
>>>>> Hi,
>>>>>
>>>>>
>>>>> On 04/24/2012 10:02 AM, Xin Tong wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> Hello
>>>>>>
>>>>>> I am wondering that can gdb insert hardware watch point for read to
>>>>>> the watched memory ? can other debugger do that ? is it supported in
>>>>>> hardware watchpoint ?
>>>>>>
>>>>>> Thanks
>>>>>>
>>>>>> Xin
>>>>>>
>>>>>
>>>>> Read watchpoints are hard to implement as soft-watchpoints, so they're
>>>>> usually implemented as hardware watchpoints.
>>>>
>>>>
>>>>
>>>> software write watchpoint is easier to implement ? do not software
>>>> watchpoint need to watch all memory accesses (read and write) to a
>>>> memory location in software ?
>>>
>>>
>>>
>>> Writes are "easier" to implemenet due to the fact that they usually
>>> change
>>> memory contents. Reads don't.
>>
>>
>> Either way, do not gdb need to instrument every memory accesses if a
>> software watch point is used ?
>
>
> It does things lazily. It single-steps instruction-by-instruction and checks
> for change of contents.

ok, i see. so it  does not dissasmble the instructions.  but for read,
it may need to disassemble the instructions which makes it harder to
implement.
>
>
>>>
>>>
>>>>
>>>>>
>>>>> If a debugger knows how to properly set the bits in the hardware,
>>>>> either
>>>>> through ptrace or other means, it can configure a read/write/read-write
>>>>> watchpoint. Depends on hardware support really.
>>>>
>>>>
>>>>
>>>> do you know whether this is supported on the x86 chips ? is thee a way
>>>> to try this out in gdb ?
>>>
>>>
>>>
>>> All those modes should be supported in x86. Try "watch", "rwatch" and
>>> "awatch".
>>
>>
>> how are hardware watch,  rwatch and awatch implemented in gdb. make
>> call to ptrace apis and ptrace apis  program some x86 registers
>> through a driver interfaces ?
>>
>
> ptrace calls really. GDB's x86 backend knows how to set those bits.
>
>>>
>>>>
>>>>>
>>>>> Regards,
>>>>> Luis
>>>
>>>
>>>
>