Some questions w.r.t remote watchpoint support in i386

[Wu Zhou <woodzltc@cn.ibm.com>]

Hello all,

I am playing with remote debugging on i386 platform.  At the very first, I 
start gdbserver and gdb on two different i386 machines.  In this configuration, 
local gdb client can connect to remote gdb server.  But after I set a 
breakpoint and issue "cont" command, remote debuggee get a SIGV error.

Then I start gdbserver and gdb on the same machine.  There is no such 
error any more.  And it seems that most gdb commands work ok.  But I 
encounter some problems with h/w watchpoint operations.  I did some 
search, and don't find much information about my problems.  So I choose to 
ask here:

1. The first error I encounter is: "Couldn't write debug register: No such
process". 

After some tracking, I found gdb client is calling target_insert_watchpoint
defined in config/i386/nm-i386.h, instead of the remote_insert_watchpoint 
defined in remote.c

I guess this is a more general problem.  Whenever you change target under 
i386 platform, you will always get the i386_insert_watchpoint (and other 
three target operations) executed.

Maybe we can put these target operation into the target_ops, instead of 
defining them as macro in config/i386/nm-i386.h?  

Any comments on this thought?  Thanks.

2. Following the above thought, I comment out these four macros in 
config/i386/nm-i386.h, and add them into i386-linux-nat.c

This time, local gdb will call remote_insert_watchpoint indeed, but I get 
another error:

 Can't clear hardware watchpoints without the 'Z2' (write-watchpoint) packet.

"set debug remote 1" tell me that Packet Z2 (write-watchpoint) is NOT 
supported

I tried to use "set remote  write-watchpoint-packet 1", but it won't work 
either:

 Enabled packet Z2 (write-watchpoint) not recognized by stub

my question here is:

What is the stub refered to here?  Is it the gdbserver?  And how can 
gdbserver determine which packat he will support, which won't?  

As far as I understand,  now that the underlying target support hardware 
watchpoint, there is not any practical obstacle for gdbserver on i386 to 
support this kind of packets.  If I am wrong, please forgive me and 
correct me. (I am no expert in this field anyway.  :-)

3. I also have a question about the overall status of gdb's support for 
remote debugging on different platforms.

The available documents (gdb manual, gdb internal) don't mention much 
information about this.  I had thought that all commands avaialbe in local 
gdb will also work in remote mode.  But the practice prove that I am 
wrong.  Playing with remote debugging always make me feel a little 
frustrated.  (I am not very sure what other feels.  I guess embedded 
people will feel more comfortable. :-)

Regards
- Wu Zhou