linux-kernel - Re: [Patch 01/11] Introducing generic hardware breakpoint handler interfaces

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Date:	Fri, 20 Mar 2009 19:30:58 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@....ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Maneesh Soni <maneesh@...ibm.com>,
	Roland McGrath <roland@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [Patch 01/11] Introducing generic hardware breakpoint handler
	interfaces


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

> > + * Kernel breakpoints grow downwards, starting from HB_NUM
> > + * 'hbkpt_kernel_pos' denotes lowest numbered breakpoint register occupied for
> > + * kernel-space request
> > + */
> > +unsigned int hbkpt_kernel_pos;
> 
> This doesn't make much sense.  All you need to know is which 
> registers are in use; all others are available.
> 
> For example, suppose the kernel allocated breakpoints 3, 2, and 1, 
> and then deallocated 2.  Then bp 2 would be available for use, 
> even though 2 > 1.

it's a high/low watermark mechanism. Yes, it's not an allocator that 
can allocate into a debug registrs 'hole', but it is a simple one 
that matches current hardware breakpoint usages and enables the 
kernel to utilize them as well - and keeps all the code simple.

	Ingo
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