kernel memory checks on boot vs. boot time

[Matthew Dillon]

    In DFly I just changed to code to check one word per 64KB instead of
    one word per 4KB, which cuts the time down by a factor of 16.  At least
    some basic 'is this actually memory' test should probably still be done.
    Probing 64G of ram was really annoying before I made that change.

    Another solution would be to use a much larger mmu mapping, like a DMAP
    style mapping.  Iterating the dram isn't going to be fast no matter what
    since each test requires a full RAS cycle anyway.  I didn't try this
    approach but it would be easier to code (treating the page directory
    entry as a mapping cache for multiple tests).

    Adding programmable bells and whistles would be overkill, and high-end
    machines shouldn't require a tunable bypass to boot reasonably anyhow.

						-Matt