x86/xen: stop assuming kernel memory loading order in PVH

[royger]

Summary

Do not assume that start_info will always be loaded at the highest
memory address, and instead check the position of all the loaded
elements in order to find the last loaded one, and thus a likely safe
place to use as early boot allocation memory space.

Reported by: markj, cpercival
Sponsored by: Citrix Systems R&D

[markj]

sys/x86/xen/pv.c

154

max() is not really appropriate here. It only compares a pair of unsigned ints. (Yes, it's too confusing.) The MAX() macro is a better alternative. I think this happens to work because the addresses involved are small, but it'd still be better to switch away from its use.

176

Extra newline.

[royger]

sys/x86/xen/pv.c

154

Yes, all addresses should be below 4G, and hence fit in an unsigned int. That mess with max() is confusing, will switch to MAX() instead.

[royger]

• Use MAX() instead of max().
• Remove extra newline.