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

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Message-Id: <1237002455.25062.83.camel@pasglop>
Date:	Sat, 14 Mar 2009 14:47:35 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	prasad@...ux.vnet.ibm.com
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Roland McGrath <roland@...hat.com>
Subject: Re: [patch 02/11] x86 architecture implementation of Hardware
 Breakpoint interfaces

On Wed, 2009-03-11 at 23:11 +0530, K.Prasad wrote:
> With FCFS or an allocation mechanism without the (un)installed()
> callbacks we'd lose the ability to record requests and service them
> later when registers become availabile.
> 
> Say when (un)installed() callbacks are implemented for the proposed
> ftrace-plugin to trace kernel symbols, they can automatically stop/start
> tracing as and when registers become (un)available. This can be helpful when
> we wish to profile memory access over a kernel variable for a long duration
> (where small loss of tracing data can be tolerated), while the system would
> permit simultaneous user-space access (say a GDB session using 'hbreak').
> 
> Are we fine with disallowing such usage, which if done will let the requester
> of the breakpoint register 'poll' periodically to check availability.

Is that such a big deal ? Can't we just have the kernel degrade to
classic SW breakpoints ?

Smells like overengineering to me ...

Ben.


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