use PAE format for the page tables, but does not incur other
consequences of the full PAE config. In particular, vm_paddr_t and
bus_addr_t are left 32bit, and max supported memory is still limited
by 4GB.
The option allows to have nx permissions for memory mappings on i386
kernel, while keeping the usual i386 KBI and avoiding the kernel data
sizing problems typical for the PAE config.
Intel documented that the PAE format for page tables is available
starting with the Pentium Pro, but it is possible that the plain
Pentium CPUs have the required support (Appendix H). The goal is to
enable the option and non-exec mappings on i386 for the GENERIC
kernel. Anybody wanting a useful system on 486, have to reconfigure
the modern i386 kernel anyway.
Discussed with: alc, jhb
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
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