Re: i386 Hardware watchpoints using kgdb-2-2.6.14.tar.gz

Thanks, Dennis. Let's stick to "debug register allocation" as suggested by 
you.

kgdb_set_hw_break definition contains only one function parameter, which is 
address. Current i386.patch includes a kgdb_set_hw_break function which 
always sets a data watchpoints. That's the reason you couldn't get execution 
watchpoints as you have mentioned in an earlier email.

kgdb_ops->set_hw_breakpoint has three parameters, which is the correct way of 
doing it.

-Amit

On Thursday 12 Jan 2006 8:52 am, Dennis W. Tokarski wrote:
> Hey there Amit,
>
> Amit Kale wrote:
> > On Saturday 07 Jan 2006 3:46 am, Dennis W. Tokarski wrote:
>
> [snip]
>
> >>2.6.14.3 kernel that I'm using.  I did not include the second patch
> >>which adds kwatch-points--it's a neat feature but not directly
> >>related to kgdb.
> >>
> >>Do you want a copy of the resulting debug register allocation
> >>patch?
> >
> > Not sure about this. When soon is kprobes patch expected to make it to
> > the kernel? If not soon, there isn't any point in using that tmechanism
> > as we can survive with some minimal code for that.
>
> Is that a change of position?  I incorporated the patch (which I should
> probably have called a "debug register allocation" patch rather than
> a "kprobe" patch) into my work mainly because of your earlier reply
> to Tom's posting on 12/16/05:
>
> =============
>
> Amit Kale wrote:
>  > That's a good scheme. We definitely want to use it when enabling
>  > watchpoints in KGDB. Even if it gets accepted in the kernel in a
>  > different form or some other scheme gets accepted, we can quickly change
>  > our code base accordingly.
>  >
>  > -Amit
>
> =============
>
> I would advocate keeping it.  Certainly something like it becomes necessary
> as soon as anything other than kgdb wants to use the hardware debug
> registers in the kernel.  For anyone debugging a complex problem in user
> space and the kernel concurently (and I've been there many times) it's
> necessary as well to keep user space gdb and kgdb from conflicting.
>
> The patch is quite minimal, being largely confined to its own header file
> and one arch-specific source file.  It touches the kernel elsewhere only
> where it's unavoidable, and even then only a couple of lines here and
> there.
>
> I don't see how it could be reduced without breaking something.
>
>
>
> [snip]
>
> >>Is one of those mechanisms deprecated? I chose for the moment to
> >>use the call through kgdb_ops.
> >
> > kgdb_ops function pointers are deprecated. We should be using weak
> > mechanisms only. set_breakpoint should also be changed this way.
> >
> > Milind, can you do the set_breakpoint change?
>
> OK, I'll do it that way.  I've captured the essentials of Milands
> patch and will follow that example for the hw_break/watch stuff.
>
> >>OK, I lied, there is actually a third issue.  You can't, in
> >>kernel/kgdb.c, just pass the ASCII type code from the Z packet
>
> [snip]
>
> > I would prefer changing the type of second parameter of set_hw_breakpoint
> > to "const char bptype". gdb remote protocol defines the values bptype can
> > take ('0-4'). Hence they are stable, besides they allow flexibility wrt.
> > achitecture.
> > -Amit
>
> Noted. I'll do that.
>
>    --Dennis
>
> >>   --Dennis
> >>
> >>Tom Rini wrote:
> >>>On Fri, Dec 16, 2005 at 10:39:42AM +0530, Amit Kale wrote:
> >>>>Dennis,
> >>>>
> >>>>That would be splendid. When a watchpoint is required, no other feature
> >>>>can do the job!
> >>>
> >>>Note that to do this correctly, you have to be aware of other potential
> >>>users.  Prasanna Panchamukhi of the kprobes project forwarded me some
> >>>code that would do this and I'll send that along to the list
> >>>momentarily.
> >>>
> >>>>-Amit
> >>>>
> >>>>On Friday 16 Dec 2005 12:41 am, Dennis W. Tokarski wrote:
> >>>>>Hi,
> >>>>>
> >>>>>Amit Kale wrote:
> >>>>>>On Thursday 15 Dec 2005 8:40 pm, Tom Rini wrote:
> >>>>>>>On Fri, Dec 09, 2005 at 12:42:39PM +0530, Amit Kale wrote:
> >>>>>>>>On Thursday 08 Dec 2005 11:13 pm, Dennis W. Tokarski wrote:
> >>>>>
> >>>>>[snip]
> >>>>>
> >>>>>>>I'm fairly certain we don't support watchpoints, and I don't
> >>>>>>>_think_ we ever did on i386 (we may have on ia64 at some point).  If
> >>>>>>>the packet isn't [zZ][01] we relpy back to GDB that it's
> >>>>>>> unsupported.
> >>>>>
> >>>>>True. But the first problem was gdb wasn't sending out Z packets
> >>>>> anyway. I have a temporary hack to fix that, but gkdb still doesn't
> >>>>> set an execution break point with Z1. It seems instead to set a data
> >>>>> access watchpoint.  As I mentioned before, I was able to manually
> >>>>> send Z packets using the maintenance command and got the same result.
> >>>>>
> >>>>>So, kgdb isn't setting the watchpoint up even when it gets the
> >>>>>right packets.  btw, I'm working exclusively on i386, so that's
> >>>>>all I can speak to.
> >>>>>
> >>>>>Anyway, what I propose to do is look back at an earlier kgdb
> >>>>>which did to all those things correctly--I've got a 2.4.18
> >>>>>kernel here set up that way--and bring the code forward into
> >>>>>the current kgdb for i386.  As part of that I'll handle the
> >>>>>additional Z packets.
> >>>>>
> >>>>>And there's still the problem with gdb itself, and why they
> >>>>>broke Z packets, but I'll take that up on the gdb list.
> >>>>>
> >>>>>>We didn't have support for official version of gdb watchpoints,
> >>>>>> though one could send "y" packets using "maintenance packet"
> >>>>>> commands. It worked. That was a couple of years ago, I believe. I
> >>>>>> can't recall when we removed support for y packets. Present code
> >>>>>> doesn't support it for sure.
> >>>>>>
> >>>>>>-Amit
> >>>>>
> >>>>>I was certainly working with the macros back in the 2.4.18 days,
> >>>>>but this is the first time I've used kdgb in 2.6.  A lot has
> >>>>>changed, not just with kgdb.
> >>>>>
> >>>>>  --Dennis
> >>>>>
> >>>>>
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