Re: [RFC] Unified watchpoints for x86 platforms

[Mark Kettenis <kettenis@wins.uva.nl>]

Eli Zaretskii <eliz@delorie.com> writes:

> While writing the code for the x86 watchpoint support, a question
> popped up regarding the Global Enable and Local Enable flags in the
> DR7 (Debug Control) register.
> 
> For those who might not remember what those flags are: Each
> hardware-assisted breakpoint or watchpoint should be enabled by
> setting bits in the DR7 register.  A watchpoint can be enabled locally
> or globally, depending on which bit in DR7 is set for that watchpoint.
> A watchpoint that is enabled locally will only break if it is hit in
> the current task.  A watchpoint that is enabled globally will break in
> any task.

Makes me wonder if global watchpoints would really work if hardware
task switching isn't used.

Anyway, I guess most x86 OS'es wouldn't allow any real global
watchpoints.  For Linux it doesn't matter whether you set the local or
the global bit.

> Now, I understand that, by and large, x86 platforms enable watchpoints
> locally, to avoid spurious traps triggered by other processes.  If
> that is true, should the code I write use the Local Enable flag
> unconditionally?  Or perhaps it would be useful to define an API that
> enables a target to control whether the global or local enable flag is
> used?

In principle a GDB command to control whether to enable watchpoints
locally or globally wouldn't be a bad idea.  But I don't know any
targets where this would be useful.  So if it isn't useful for DJGPP
either, I wouldn't bother.  A comment about the issue at the right
spot would be nice though.

Mark