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SubjectRe: [patch 02/11] x86 architecture implementation of Hardware Breakpoint interfaces

* Alan Stern <stern@rowland.harvard.edu> wrote:

> On Sat, 14 Mar 2009, K.Prasad wrote:
>
> > Here's a summary of the intended changes to the patchset, which I hope
> > to post early the following week. It tears down many features in the
> > present submission (The write-up below is done without the benefit of
> > actually having run into limitations while trying to chisel out code).
> >
> > - Adopt a static allocation method for registers, say FCFS (and perhaps
> > botton-up for user-space allocations and the reverse for
> > kernel-space), although individual counters to do book-keeping should also
> > suffice.
>
> You can't enforce bottom-up allocation for userspace breakpoint
> requests. [...]

That's not the point.

The point is to offer a reasonable and simple static allocator
that will work fine with usual gdb usage. If something takes
away db4 that's as if user-space took away all registers - tough
luck.

You are trying to put complexity into a situation that is not
schedulable hence not resolvable _anyway_. There's just 4 debug
registers, not more. If the combined usage goes above four
someone will lose anyway - even with your allocator.

With my proposal the 'loss' can indeed come sooner if user-space
took db4 and there's nothing left for the kernel anymore - but
that's just an uninteresting special case that wont occur with
typical debug-register usage.

If it ever causes problems seriously _then_ will be the time to
consider "is it worth adding a more complex, dynamic allocator
for debug registers". Not now. This stuff is currently
over-designed and not acceptable to me in its current form.

Ingo


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