Re: [PATCH]: Make Linux use the new unified x86 watchpoint support

[Mark Kettenis <kettenis@science.uva.nl>]

   Date: Tue, 27 Mar 2001 08:55:00 +0200 (IST)
   From: Eli Zaretskii <eliz@is.elta.co.il>

   On Mon, 26 Mar 2001, Michael Snyder wrote:

   > Guys, this implementation has problems.  You have it hard-coded
   > so that on a linux host, it unconditionally calls native linux
   > methods involving ptrace to get the debug registers.  This breaks
   > very badly if you're using a native linux host to debug a remote
   > i386 target.

Just like the old implementation.  I suspect the old implementation
just happened to work because it didn't do any strict error checking.

The fundamental problem is that the watpoint-stuff isn't part of the
target vector.

   Sorry, I'm not following.  The watchpoint-related macros are defined
   on files which should be used only with native debugging (i386-nat.c
   and nm-i386.h).  On top of that, the macros only get exposed if the
   port defines I386_USE_GENERIC_WATCHPOINTS.  A port which doesn't want
   that should get rid of the watchpoints for free, by simply not
   defining I386_USE_GENERIC_WATCHPOINTS.

   Which part of the above misfires, and why?

It used to be possible to debug a remote i386 target with a native
Linux/x86 debugger.

   > Seems to me, what you need to do is add these debug registers to the
   > reg cache, and treat them like ordinary registers.

   This possibility has been discussed back in November, but the
   conclusion was that it's not a good idea.  I don't remember the
   details, but the reasons had something to do with threads and how
   the register cache is used in conjunction with threads.  (I can dig
   out the URLs of the relevant messages, if you want to read them.)

I suggested doing this, but several people objected to exposing the
debug registers in this way.  Threads have nothing to do with it
(actually doing the correct thing for threads would make it easier if
the debug registers would be part of the register cache).

   > Then you can just use the ordinary read_register interface to get
   > them, and remote.c will do the right thing for you (assuming the
   > target knows about these extra registers).

   Mark explicitly didn't want the watchpoint code to be in the
   target-depenent files, so watchpoints cannot currently work for remote
   targets.

Actually, our problems would only be bigger if the watchpoint code was
part of the target-dependent files.  But if we add the debug registers
to the register cache and add the watchpoint stuff to the target
vector, only a few changes are necessary to make the watchpoint code
ready for a move to i386-tdep.c.

Mark