Re: Hardware watchpoint for read

[Luis Gustavo <luis_gustavo@mentor.com>]

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.

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