linux-kernel - Re: [patch 02/11] x86 architecture implementation of Hardware Breakpoint interfaces

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Message-ID: <Pine.LNX.4.44L0.0903141157320.8138-100000@netrider.rowland.org>
Date:	Sat, 14 Mar 2009 12:10:51 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Ingo Molnar <mingo@...e.hu>
cc:	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch 02/11] x86 architecture implementation of Hardware
 Breakpoint interfaces

On Sat, 14 Mar 2009, Ingo Molnar wrote:

> 
> * Alan Stern <stern@...land.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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