Re: Using XML in GDB?

[Daniel Jacobowitz <drow at false dot org>]

On Thu, Jan 26, 2006 at 09:36:33PM +0100, Mark Kettenis wrote:
> > Date: Thu, 26 Jan 2006 00:57:44 -0500
> > From: Daniel Jacobowitz <drow@false.org>
> > 
> > Hi folks,
> > 
> > I've spent the last week and a half working on the "target available
> > feature" interface that I described on this list last May.  A big chunk
> > of the last two days has been spent trying to nail down a useful format
> > to store (in files) and transfer (over the remote protocol) descriptions of
> > remote "features", especially their register sets.
> > 
> > At first I was primarily focused on compactness.  But I've got a pretty good
> > handle on that problem now; a well-defined naming scheme and some caching,
> > and the size of the data is no longer a major concern.
> 
> Does that hold for slow serial links too?

Yes.  Here's how it works in my current design:

- GDB connects to the target.
- GDB queries the target for its features.
- The target replies briefly (basically just name and a base integer
  for register numbering).
- GDB looks up the features by name in the set of features it
  recognizes.
- GDB queries the target for any features it does not recognize.
- GDB caches the queried features (with some exceptions).  The cache
  could be either in RAM, or on disk if desired and configured.

Suppose a verbose, well-documented register description is 50K - my
current (non-XML, but also non-verbose) descriptions are only about 2K. 
Over a 9600 baud serial link, which is about the slowest we're likely
to encounter, this could take about a minute to transfer - the first
time.  That would be a good motivation to use the on-disk caching
and maybe a progress indicator.

Most of my current debug targets have higher bandwidth links than that,
although often with a very high latency; I've got some tricks up my
sleeve for reducing the impact of latency, too (coming soon to a gdb@
near you).

> > And we get all sorts of things for free; for instance, UTF-8, which will be
> > handy if someone ever wants to include internationalized descriptions in the
> > target description.
> > 
> > Does anyone have a good reason why GDB should not make use of this
> > well-standardized format instead of inventing additional ad-hoc formats?
> 
> No objection to using a well-standardized format, but that probably
> only makes sense if you're not going to write your own parser.  And I
> really would like to avoid making GDB dependent on a library that
> isn't standardly available on all systems we support.

I definitely have no intention of either writing my own XML parser or
introducing an external dependence.  I am tentatively planning to
import one of {expat, libxml2} into src/ and arranging for it to be
built as necessary.

Sadly there does not seem to be a GNU implementation of an XML parser
with C bindings; I only found Java bindings.  If anyone knows to the
contrary, please holler.

-- 
Daniel Jacobowitz
CodeSourcery