Re: Fw: [ppc-linux-nat]: set access flag for h/w watchpoint even if it is only read or write (fwd)

[Daniel Jacobowitz <drow@false.org>]

On Wed, Jul 12, 2006 at 09:15:48PM +0300, Eli Zaretskii wrote:
> > Date: Wed, 12 Jul 2006 13:31:25 -0400
> > From: Daniel Jacobowitz <drow@false.org>
> > Cc: eliz@gnu.org, gdb-patches@sourceware.org
> > 
> > First, there's the question of targets which can't set true "read"
> > watchpoints, but can set "access" watchpoints.  Right now the x86
> > simply accepts a request to insert a read watchpoint, and generates an
> > access watchpoint instead.  How about if we changed it to refuse to
> > insert read watchpoints, changed breakpoint.c to attempt an access
> > watchpoint if inserting a read watchpoint fails, and then store in the
> > breakpoint which type was inserted?
> 
> Sorry, I don't understand your plan.  How is it different from what
> GDB does now (silently replace read with access watchpoint)?  And what
> type will we store when the user says "rwatch"?

It's a layering change.

Right now the i386 target says "you asked me to insert a read
watchpoint.  OK!  I've done it" and sneakily inserts an access
watchpoint instead.  That means higher layers can't tell that it's
going to fire unexpectedly.

If the i386 target instead said "Sorry, I can't insert a read
watchpoint" and the higher layer said "well, how about this - can
you insert an access watchpoint?" then it would be able to handle and
skip the unexpected firing.

> > Second, there's the question of which sort of watchpoint we've hit. If
> > the target could tell us, say, as a return value from
> > target_stopped_data_address, then we could only check read watchpoints
> > when a read watchpoint triggers.  But I think this is a smaller issue;
> > at worst we might report that both a write and read watchpoint had
> > triggered when really only a write watchpoint had.
> 
> I think this is bad: if a user sets two different watchpoints at the
> same address, she wants each watchpoint to fire when its specific
> conditions are met.
> 
> My philosophy here is that watchpoints are a kind of silver bullet:
> you use them when you have almost no idea who could touch the data in
> question.  So watchpoints should perform as close to the spec as
> possible, because the user is already in a very confused state;
> confusing her even more with imprecise watch reports would be a bad
> mantra, IMO.

Yes, I know.  I'm just trying to be pragmatic.  The two items in my
mail are different problems and will require different solutions;
fixing one is better than fixing neither IMO, and this is the harder
one to fix.  It would be a shame to block a fix for one on a fix for
the other.

There's probably more than one target that can't tell us which sort of
watchpoint triggered.  In fact, I don't have first hand knowledge of
any target which could tell us which triggered.  You might be able to
distinguish if you used one set of watchpoint registers to set a write
watchpoint and another to set a read watchpoint - but that's awfully
wasteful of watchpoint registers.

-- 
Daniel Jacobowitz
CodeSourcery