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