Re: Hardware watchpoint for read

[Luis Gustavo <luis_gustavo@mentor.com>]

On 04/24/2012 12:06 PM, Xin Tong wrote:
> 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.

Yes, this is the problem with read watchpoints implemented as software 
watchpoints. It's hard to determine a trigger without making it 
painfully slow, and the debugger needs knowledge about the arch's 
instructions.

>>
>>
>>>>
>>>>
>>>>>
>>>>>>
>>>>>> 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
>>>>
>>>>
>>>>
>>