Re: [PATCH] Fix displaced stepping for remote targets

[Luis Gustavo <luis_gustavo@mentor.com>]

On 03/20/2012 01:34 AM, Yao Qi wrote:
> On 03/20/2012 10:02 AM, Luis Gustavo wrote:
>> If a target supports hw single-stepping for displaced stepping, GDB
>> should just send a vCont;s packet to tell the target to step a single
>> instruction. In my case, GDB was always sending a vCont;c instead.
>
> On some arch, the original instruction may be translated to more than
> one instructions, and copied to scratch pad.  So, we can't do
> single-step in scratch pad.

During a few tests x86 seems to do single-stepping in the scratch pad 
when using displaced stepping, or at least it sends the correct vCont;s 
packet.

>
> Even the case that one instruction is generated/copied to scratch pad
> can be optimized in your approach, but not sure it breaks
> software-single-step arch or not.
>

In my case GDB thinks there's a breakpoint right at PC since PC points 
to the old location. GDB just continues through vCont;c. This does not 
work as execution will just re-start from the scratch pad area and will 
cause unpredictable behavior.

Maybe we need a better mechanism for this then.

Luis