Re: [RFC] GDB patches for hw watchpoints - revised

[Daniel Jacobowitz <drow at false dot org>]

On Tue, Dec 13, 2005 at 02:10:03PM +0800, Wu Zhou wrote:
> 2. The second one need Anton's patch, which changed three lines in 
> arch/ppc64/mm/fault.c:

> With this patch, I can use PTRACE_GETSIGINFO to get the stopped data 
> address (it is in siginfo.si_addr).  But one problem is that 
> to_stopped_by_watchpoint will call PTRACE_GETSIGINFO first to determine if 
> the stop is caused by watchpoint.  And another problem is that gdb need to 
> single step the process to execute current instruction when a watchpoint 
> is hit.  This will again drop into bpstat_stop_status, which will call 
> stopped_by_watchpoint and thus call PTRACE_GETSIGINFO again.  
> 
> I take a look at IA64's code, it set the dd bit of IA64_PSR_REGNUM, which
> will disable the watchpoint for the next instruction.  But it seems that 
> ppc don't have such a way.  Do we have any workaround for this?  

Could you cache the stopped data address when we are stopped by a
watchpoint, and then return the cached one if we aren't stopped by a
watchpoint any more but were the previous instruction?

I realize this is gross; the entire "step, then check the stopped data
address" is a bad idea, in my opinion.

> 3. The third one is a little tricky.  Now that ppc has at most 1 DABR. So 
> I can set the stopped_data_address to the data address when we set the 
> watchpoint (in ppc_linux_insert_watchpoint).  Everytime 
> target_stopped_data_address is called, the breakpoint is either read or 
> access, so it is already clear that it is stopped by watchpoint.  Then 
> this trick seems to make sense, right?

Weren't there other PPCs with more than one though?

> I had tested the above three methods.  The first one works ok when the 
> data breakpoint is aligned by 8 bytes.  The third one works ok for both 
> aligned and non-aligned data breakpoint.  For the second one, I don't know 
> how to work around the extra PTRACE_GETSIGINFO call caused by the single 
> step yet.  But if I reserver the stopped_data_address when we first 
> hit watchpoint, and store it back when I call ppc_linux_stopped_data_address. 
> I can make rwatch and awatch to work as expected.

... right, I guess I should have read your whole message first!  That's
my preferred option if we can find a clean way to do it.  I thought
there was a target with an example of this, but I can't find it.  I'm
not sure I understand why ia64 needs to disable the watchpoint, though.

-- 
Daniel Jacobowitz
CodeSourcery, LLC