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Date: Tue, 9 Mar 2010 10:57:45 +1100
From: David Gibson <dwg@au1.ibm.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Subject: Re: [Patch 1/1] PPC64-HWBKPT: Implement hw-breakpoints for PPC64
Message-ID: <20100308235745.GE4993@yookeroo>
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Cc: Michael Neuling <mikey@neuling.org>,
	Benjamin Herrenschmidt <benh@au1.ibm.com>, shaggy@linux.vnet.ibm.com,
	Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>,
	Will Deacon <will.deacon@arm.com>, linuxppc-dev@ozlabs.org,
	Alan Stern <stern@rowland.harvard.edu>, paulus@samba.org,
	"K.Prasad" <prasad@linux.vnet.ibm.com>, Roland McGrath <roland@redhat.com>
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On Fri, Feb 26, 2010 at 02:58:12AM +0100, Frederic Weisbecker wrote:
> On Mon, Feb 22, 2010 at 06:47:46PM +0530, K.Prasad wrote:
[snip]
> > > Oh, why does ptrace use a one-shot behaviour in ppc? Breakpoints
> > > only trigger once?
> > > 
> > 
> > Yes, ptrace breakpoints on PPC64 are designed to trigger once and this
> > patch retains that behaviour. It is very convenient to use one-shot
> > behaviour on archs where exceptions are triggered-before-execute.
> 
> Ah, Why?

Because otherwise you have jump through some tricky hoops so that when
gdb (or whatever) resumes after the breakpoint, you don't immediately
retrap in the same place.  I gather x86 has hardware assistance to do
this, but powerpc doesn't.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson