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

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Message-ID: <Pine.LNX.4.44L0.0903101017040.2962-100000@iolanthe.rowland.org>
Date:	Tue, 10 Mar 2009 10:19:47 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Ingo Molnar <mingo@...e.hu>
cc:	prasad@...ux.vnet.ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Roland McGrath <roland@...hat.com>
Subject: Re: [patch 01/11] Introducing generic hardware breakpoint handler
 interfaces

On Tue, 10 Mar 2009, Ingo Molnar wrote:

> * prasad@...ux.vnet.ibm.com <prasad@...ux.vnet.ibm.com> wrote:
> 
> 
> > +static u8			tprio[HB_NUM];	/* Thread bp max priorities */
> > +LIST_HEAD(kernel_bps);			/* Kernel breakpoint list */
> > +static LIST_HEAD(thread_list);			/* thread_hw_breakpoint list */
> > +DEFINE_PER_CPU(struct cpu_hw_breakpoint, cpu_bp);

If nobody minds, I'll answer some of these questions on Prasad's behalf 
because they address parts of the code that were written before he took 
over the project.

> hm, why do we need the whole 'priority' mechanism? It seems very 
> over-designed to me.

This was done at Roland McGrath's express request.  We should see what 
he has to say about it.

> The likelyhood of both user-space and kernel-space to use 
> hw-breakpoints is very low to begin with. And if they use them, 
> the likelyhood of there being more than 4 debugregs required in 
> the same context is even lower.

Not all architectures have 4 debug registers.  Most have only one.

> If that happens we shouldnt try to be too smart about them - 
> just override user-space ones with kernel space ones and that's 
> it. No explicit priorities are needed.

Roland really did not want it done this way.

Alan Stern

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