Re: Hardware watchpoint for read

[Luis Gustavo <luis_gustavo at mentor dot 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