If we're mapping something in the "legacy range" (0-1Mb), we shouldn't
expand the requested range to the entire 0-1Mb range. Typically this
is for mapping the VGA frame buffer, and some platforms support mmap of
the frame buffer but not the entire 0-1Mb range.
For example, HP sx1000 and sx2000 ia64 platforms can have memory from
0-0x9ffff, VGA frame buffer from 0xa0000-0xbffff, and memory from
0xc0000-0xfffff. On these platforms, we can't map the entire 0-1Mb
range with the same attribute because the memory only supports WB,
while the frame buffer supports only UC. But an mmap of just the
frame buffer should work fine.
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