Re: [Patch 1/1] PPC64-HWBKPT: Implement hw-breakpoints for PPC64

["K.Prasad" <prasad@linux.vnet.ibm.com>]

On Fri, Dec 18, 2009 at 12:33:20AM +0530, K.Prasad wrote:
> On Mon, Dec 14, 2009 at 11:26:26AM -0800, Roland McGrath wrote:
<snipped>
> > What remains less than clear is how preemption relates.  For any per-thread
> > hw_breakpoint, there is no high-level reason to care one way or the other.
> > The thread, its HW breakpoints, its register state including state of
> > stepping, are all part of per-thread state and no reason to do any less (or
> > more) preemption than normally happens.
> > 
> 
> I get that reasoning now...I'd been unduly worried about pre-emption
> and hence the introduction of pre-emption disabled state.
> 
> But of course, in the existing design, the per-cpu variables would be
> affected because if pre-emption was to occur. I'll see how that can be
> factored in, while retaining the abstraction provided by the interfaces.
> 
<snipped>
> 
> As stated above, I was worried about a pre-emption happening between a
> return from breakpoint exception handler and the execution of the
> causative instruction....but as I learn, it seems fine now. It is just that
> the kernel code needs to be tweaked keeping this in mind.
> 
> Thanks,
> K.Prasad
> 

Hi Roland,
	The cost of allowing pre-emption (such as storing the per-cpu
variables into per-thread structures for user-space breakpoints and
offsetting the effect of a new process between the hw-breakpoint handler
and single-step handler) appears to far out-weigh the present approach
(where pre-emption is disabled between two user-space instructions).

It is also not clear to me if disabling pre-emption for the user-space
(albeit for a very tiny time-window) is incorrect and if their side-effects
are known. If otherwise, I think we should choose to operate in pre-empt
safe mode and avoid all costs associated when done without it.

I'm eager to know what you think.

Thanks,
K.Prasad