Messages in this thread | | | Date | Sat, 14 Mar 2009 12:10:51 -0400 (EDT) | From | Alan Stern <> | Subject | Re: [patch 02/11] x86 architecture implementation of Hardware Breakpoint interfaces |
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On Sat, 14 Mar 2009, Ingo Molnar wrote:
> > * 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.
You are reading far more into my message than what I wrote.
I'm _not_ trying to put complexity anywhere. All I did was point out that Prasad was wrong to state that the kernel could adopt (or enforce) a bottom-up method for allocating debug registers for userspace breakpoints. I trust you aren't trying to imply that he really was right?
> 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.
My message didn't mention a word about more complex, dynamic allocation. Just the opposite, in fact -- because if we did virtualize the debug registers then we _would_ be able to enforce bottom-up allocation.
So in the end, you're _agreeing_ with what I wrote. And yet the tone of your reply suggests that you seemed to think that my message had some deep, hostile intent. It didn't.
Alan Stern
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