Re: Some questions about gdb's remote protocol and reverse debugging

[Julian Smith <jsmith at undo-software dot com>]

On Tue, 11 Aug 2009 16:22:30 -0700
Michael Snyder <msnyder@vmware.com> wrote:

> Julian Smith wrote:
> > Hello
> > 
> > I've been trying out gdb-cvs's remote protocol, specifically the
> > commands for reverse debugging, and i have a couple of questions that i
> > was hoping someone might be able to help me with. I'm using Linux on
> > x86-32 and x86-64.
> 
> Cool, welcome!  Join the fun.
> 
> > First, if i'm understanding things correctly, gdb appears to default to
> > software breakpoints, using the 'Z0' and 'z0' commands and, if these
> > aren't supported by the remote target, it then tries to use 'M' and 'm'
> > to write breakpoints directly into the inferior's memory.
> 
> That's right...
> 
> > Is there any way to tell gdb to try to use hardware breakpoints (with
> > the 'Z1' and 'z1' commands) before resorting to 'M' and 'm' ? [In the
> > environment i'm working in, UndoDB, hardware breakpoints are more
> > convenient because they don't require any patching up of %pc, and
> > poking breakpoints directly into memory is not supported.]
> 
> Not as such, no.  You use a different syntax to set a hardware
> breakpoint.  The command is "hbreak" instead of "break".  You
> should at least be able to try stuff out with that.
> 
> You may need to enable the Z1 packet, like this (I'm not sure
> if it's enabled by default):
> 
>    set remote haredware-breakpoint-packet 1

Ah, i hadn't noticed this command, though i have been using `hbreak'.
Thanks. I'm also trying out disabling the software-breakpoint packet,
and things seem to be working better now.

> 
> > Second, am i right in thinking that gdb does things like reverse-step
> > and reverse-next by effectively doing many reverse-stepi's (with 'bs'),
> > interleaved with 'g' commands to get the registers? If so, are there
> > any plans to try to avoid the overhead of this somehow ?
> 
> There are only the two actual reverse-execution packets -- 'bs' and 
> 'bc'.  Gdb uses them in exactly the same contexts that it would use
> 's' and 'c' if going forward.  As such, "step" is always going to be
> implemented as one or more 's' requests, whereas "next" may involve
> a mixture of 's' and 'c' requests.  The same is true going backward.

Ok.

But... isn't there always going to be a difference when going
backwards, e.g. if an instruction jumps to an address in a register,
one cannot know about this jump when stepping backwards (and so, for
example, one cannot first go back to the destination of the jump, do a
`bs' to step back over the jump, then carry on with `bc')? Perhaps this
sort of indirect jump never occurs within a function though, so is not a
problem for things like reverse-step.

Anyway, many thanks for you help, 'tis much appreciated. I'm still
tinkering with things, but i think i should be able to make progress
now.

Cheers,

- Julian

-- 
http://undo-software.com/