[PATCH 2/2] [PowerPC Book3E] Introduce new ptrace debug feature flag

[David Gibson]

On Wed, Aug 24, 2011 at 09:41:43PM -0300, Thiago Jung Bauermann wrote:
> On Wed, 2011-08-24 at 14:00 +1000, David Gibson wrote:
> > On Tue, Aug 23, 2011 at 02:57:56PM +0530, K.Prasad wrote:
> > > On Tue, Aug 23, 2011 at 03:09:31PM +1000, David Gibson wrote:
> > > > On Fri, Aug 19, 2011 at 01:23:38PM +0530, K.Prasad wrote:
> > > > > 
> > > > > While PPC_PTRACE_SETHWDEBUG ptrace flag in PowerPC accepts
> > > > > PPC_BREAKPOINT_MODE_EXACT mode of breakpoint, the same is not intimated to the
> > > > > user-space debuggers (like GDB) who may want to use it. Hence we introduce a
> > > > > new PPC_DEBUG_FEATURE_DATA_BP_EXACT flag which will be populated on the
> > > > > "features" member of "struct ppc_debug_info" to advertise support for the
> > > > > same on Book3E PowerPC processors.
> > > > 
> > > > I thought the idea was that the BP_EXACT mode was the default - if the
> > > > new interface was supported at all, then BP_EXACT was always
> > > > supported.  So, why do you need a new flag?
> > > > 
> > > 
> > > Yes, BP_EXACT was always supported but not advertised through
> > > PPC_PTRACE_GETHWDBGINFO. We're now doing that.
> > 
> > I can see that.  But you haven't answered why.
> 
> BookS doesn't support BP_EXACT, that's why I suggested this flag.

Surely you can support it with exactly the same sort of filtering
you're using for the 8-byte ranges now?

> A BP_EXACT watchpoint triggers only when there's a memory access exactly
> at the given address. It doesn't trigger when there's (for example) a
> 4-byte write at an address immediately before which also changes the
> memory contents of the byte watched by the BP_EXACT watchpoint. a ranged
> watchpoint would trigger, so the semantics are different.
> 
> As a general rule, GDB only sets ranged watchpoints and only uses
> BP_EXACT ones when the user sets a flag. I want GDB to fail when the
> user sets the flag on BookS since it can't provide the feature.

-- 
David Gibson			| I'll have my music baroque, and my code
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				| _way_ _around_!
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