linux-kernel - Re: [Patch 00/11] Hardware Breakpoint interfaces

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Date:	Fri, 24 Apr 2009 10:16:07 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
cc:	Ingo Molnar <mingo@...e.hu>,
	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@...ux.vnet.ibm.com>, Roland McGrath <roland@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [Patch 00/11] Hardware Breakpoint interfaces

On Fri, 24 Apr 2009, K.Prasad wrote:

> The arch_update_kernel_hw_breakpoints() was designed to work like this -
> it updates all registers beginning 'hbp_kernel_pos' to (HB_NUM - 1) with
> the values stored in hbp_kernel[] array.
> 
> When inserting a new breakpoint, hbp_kernel_pos is decremented *before*
> invoking arch_update_kernel_hw_breakpoints() so that the new value is
> also written onto the physical debug register.
> 
> On removal, 'hbp_kernel_pos' is incremented *after*
> arch_update_kernel_hw_breakpoints() so that the physical debug registers
> i.e. both DR7 and DR<pos> are updated with the changes post removal and
> compaction. I'm ready to make changes but don't see where the code
> actually goes wrong. Can you explain that?

I'm sorry; I misread the code in arch_update_kernel_hw_breakpoints().  
It isn't actually wrong, and you are correct to increment
hbp_kernel_pos where you do.  Your code is different from my original
version, which would update all the debug registers at once instead of
doing the kernel and userspace breakpoints separately -- that's what 
confused me.

There is one change you could make to improve the routine, however.  In 
arch_update_kernel_hw_breakpoints(), the line

	kdr7 &= ~kdr7_masks[hbp_kernel_pos];

really should be

	kdr7 = 0;

since kdr7 never contains anything other than kernel breakpoint
settings.  (You could update the comment in the preceding line as
well.)

Alan Stern

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