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Subject: Re: [Patch 0/6] [Patch 0/6] PPC64-HWBKPT: Hardware Breakpoint
	interfaces - ver VIII
From: Luis Machado <luisgpm@linux.vnet.ibm.com>
To: David Gibson <dwg@au1.ibm.com>
In-Reply-To: <20090731061013.GG3950@yookeroo.seuss>
References: <20090727001152.GA13562@in.ibm.com>
	<20090731061013.GG3950@yookeroo.seuss>
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Date: Mon, 03 Aug 2009 13:14:56 -0300
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Cc: Michael Neuling <mikey@neuling.org>,
	Benjamin Herrenschmidt <benh@au1.ibm.com>,
	linuxppc-dev@ozlabs.org, paulus@samba.org,
	Alan Stern <stern@rowland.harvard.edu>,
	"K.Prasad" <prasad@linux.vnet.ibm.com>, Roland McGrath <roland@redhat.com>
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Hi,

On Fri, 2009-07-31 at 16:10 +1000, David Gibson wrote:
> On Mon, Jul 27, 2009 at 05:41:52AM +0530, K.Prasad wrote:
> > Hi David,
> > 	I'm back with a new version of patches after a brief hiatus!
> > 
> > After much deliberation about modifying the code to change the timing of signal
> > delivery to user-space, it has been decided to retain the existing behaviour
> > i.e. SIGTRAP delivered to user-space after execution of causative instruction
> > although exception is raised before execution of it.
> 
> Ok.  Except, presumably for ptrace, since changing that would break
> gdb.

Yes. GDB works with some assumptions in mind. Hardware breakpoints
(they're not supported right now) trigger before execution, then GDB
just removes them and stepi's until we're past the breakpoint.

As for HW watchpoints, we always stop before execution (by ppc design
presumably). GDB will check the value before the trigger, remove the HW
watch, stepi it, and will check the value again to see if it has
changed.

If we're to trigger after the data load/store has happened, we could
have breakage in GDB.

Regards,
Luis