[Xen-devel] save image file format

[John Levon <levon@xxxxxxxxxxxxxxxxx>]

I brought this up with a few people at the summit, but I thought I'd
gauge interest here.

The current format used for save files and migration has a number of
problems:

1) it's not self-identifying (word size, machine type)
2) it's unversioned
3) it misses some useful meta information (e.g. xen version)
4) it's not easily extendable
5) it needlessly uses a different format from core files, requiring a
debugger to implement both formats as a backend if support for both is
wanted

1) and 5) makes implementing a debugger that works sanely very
difficult. 2) and 4) are likely to significantly complicate things in
the future, whilst 3) makes debugging a lot less useful.

As the migration format needs modifying for HVM anyway as Intel
presented, and presumably for XML config changes, I suggest that a new
format be put in place during the 3.0.4 timeline. Are there other people
interested in such a change?

I was thinking of a simple ELF-like format. One problem is that of
migration's streaming format, but this could be dealt with as a special
case (e.g. that section's size is explicitly labelled as '0' to mean
"special case").

There also remains the question of supporting older Xens. I don't
believe that anything other than migration back-compatibility is
interesting in this case, and presumably this can be handled reasonably
easily by a slight refactoring of xc_linux_{save,restore} (on the
presumption that people will want to migrate both to and from such older
dom0's), and falling back to compat mode when the new negotation API
fails.

regards
john

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