From the Linux kernel arch/x86/kernel/cpu/mtrr/generic.c:
high_width = ffs(size_or_mask & 0xffffffffUL) - 1;
high_width becomes the bit position of the least
significant bit set in size_or_mask. Note, the least
significant bit for ffs is 1, not 0, because 0 is returned if no bit is set. The -1 is used to convert the result of ffs() to
the standard bit numbering.