Re: [PATCH] Fix read and access watchpoint can't stop

[Eli Zaretskii <eliz@gnu.org>]

> From: Hev <r@hev.cc>
> Date: Thu, 18 Jun 2020 10:05:14 +0800
> Cc: gdb-patches@sourceware.org
> 
> > >       /* We were stopped by a hardware watchpoint, but the target could
> > >          not report the data address.  We must check the watchpoint's
> > >          value.  Access and read watchpoints are out of luck; without
> >
> > The comment below the line you suggest to modify explains why we don't
> > handle access and read watchpoints here.  So I don't understand the
> > rationale for this change (it might be a good idea to explain it in
> > more detail in the log message).  What am I missing?
> >
> > Thanks.
> 
> This problem only occurs on the target that target_stopped_data_address() returns false, and
> b->watchpoint_triggered is set to  watch_triggered_unknown when watchpoint is triggered. so, In
> bpstat_check_watchpoint(), the last branch needs to be hit to enable check value:
> 
>  4961       int must_check_value = 0;
>  4962
>  4963       if (b->type == bp_watchpoint)
>  4964     /* For a software watchpoint, we must always check the
>  4965        watched value.  */
>  4966     must_check_value = 1;
>  4967       else if (b->watchpoint_triggered == watch_triggered_yes)
>  4968     /* We have a hardware watchpoint (read, write, or access)
>  4969        and the target earlier reported an address watched by
>  4970        this watchpoint.  */
>  4971     must_check_value = 1;
>  4972       else if (b->watchpoint_triggered == watch_triggered_unknown
>  4973            && b->type == bp_hardware_watchpoint)
>  4974     /* We were stopped by a hardware watchpoint, but the target could
>  4975        not report the data address.  We must check the watchpoint's
>  4976        value.  Access and read watchpoints are out of luck; without
>  4977        a data address, we can't figure it out.  */
>  4978     must_check_value = 1;
> 
> However, the b->type is the actual watchpoint type, bp_hardware_watchpoint for write-only,
> bp_read_watchpoint for read-only, and bp_access_watchpoint for read-write, so it will not hit when read-only
> and read-write watching.
> 
> On mips we don't know the low order 3 bits of the data address, so we must return false in
> stopped_data_address(). This is why we have not found the problem on other platforms (x86/Aarch64/...).

I understand, but how will this work on MIPS, given that
is_hardware_watchpoint allows any type of watchpoints?  How will GDB
know which watchpoint triggered if there's no address?

And what is the value of bs->breakpoint_at, and how is it different
from b->type?

Thanks.