As has been requested, this series adds new overlap check functions to
the qcow2 code. My local branch is called "qcow2-improved-overlap-v1",
but I am not so sure whether it is actually an improvement; that is left
for you to decide, dear reviewers.
See patch 1 for an explanation of why this series exists and what it
does. Patch 1 is basically the core of this series, the rest just
employs the functions introduced there.
I have yet to do benchmarks to test whether this series actually
improves things, but judging from the iotests it at least does not slow
things down (which it did at one time during development, particularily
test 044 is good for testing this, so this actually has some
significance to it).
In a later patch, we may want to change the meaning of the "constant"
overlap checking option to mean the same as "cached", which is
everything except for inactive L2 tables. This series does make
checking for overlaps with inactive L2 tables at runtime just as cheap
as everything else (constant time plus caching), but using these checks
means qemu has to read all the snapshot L1 tables when opening a qcow2
file. This does not take long, of course, but it does result in a bit of
overhead so I did not want to enable it by default.
I think just enabling all overlap checks by default after this series
should be fine and useful, though.