Before proceeding, it is a good idea for a kernel to perform some basic validation
and check that it is booting in a compatible version of Xen. The magic
value in the structure is a string of not more than 32 characters of the form ???Xenversion.
sub version??? and can be used for this purpose. In addition to checking
that the version of Xen is supported, this ensures that the start info page has been
mapped correctly. If it doesn??™t start with ???Xen-???, something is seriously wrong,
and the best thing to do is abort. Xen guarantees backward compatibility within
major versions, although features may be added between minor versions. You
should check that the major version is equal and the minor version is not lower
than the version on which the kernel was tested.
When you are happy that you are running in a supported Xen environment,
booting can proceed. A kernel typically needs to know early on how much RAM
3.1. Retrieving Boot Time Info 49
it has available to it, and how many CPUs. The number of pages is provided by
the start info structure in the nr pages element.
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