On Tue, Aug 04, 2009 at 02:23:16AM +0530, K.Prasad wrote:
> On Fri, Jul 31, 2009 at 04:10:13PM +1000, David Gibson wrote:
> > On Mon, Jul 27, 2009 at 05:41:52AM +0530, K.Prasad wrote:
>
> <edited>
>
> > > Reasons
> > > --------
> > > - Signal delivery before execution of instruction requires complex
> > > workarounds
> > > - One of the plausible workarounds is a two-pass hw-breakpoint handler
> > > which
> > > delivers the signal after the first pass (with the breakpoints enabled).
> > > In the second pass, it follows the existing semantics of
> > > disable_hbp-->enable_ss-->single_step-->disable_ss-->enable_hbp.
> >
> > Yes, that's the only way I can see to do it.
> >
> > > - Possibility of nested exceptions is a problem here.
> >
> > Ok, why?
> >
>
> Reason as described in the para below.
>
> > > - Proper identification of a second-pass of first exception and a new
> > > nested
> > > exception is difficult. Possibility of stray exceptions due to accesses
> > > in
> > > neighbouring memory regions of the breakpoint address further
> > > complicates it.
>
> To elaborate, consider a case where a user-space address 'x' is
> monitored for read or write, and the following happens (assume the
> existence of the two-pass method for signal delivery).
>
> - Instruction 'i' attempts to read/write in address 'x'
> - hw-bkpt exception generated (pass I)
> - Signal generated and hw-bkpt exception returns to user-space
> - Signal is handled before 'i' is executed. Handler code reads/writes
> data in 'x' again. Generates nested exception.
> - hw-breakpoint handler code is unable to distinguish if the new
> exception is from signal handler (nested) or due to second-pass (as
> per design above).
Ah, ok, I understand now. Hrm. I'll have to think about this.
--
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
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