[coreboot] more smm questions

[ron minnich]

OK, I have it working. For the Q35 qemu mainboard, I can direct SMI to the
kernel. The final issue was that the existing linux trampoline can't work
at present if you have enabled NX and set the top bit of a PTE to 1, since
the trampoline doesn't enable NX correctly. Easy fix: add nonx=off to the
commandline. that's not a typo, even though one might expect it to be
nonx=on

So, it's possible to have your kernel handle SMIs and run code that
otherwise would be in ring -2.

We've been advised that the best thing to do with SMI is disable it totally
(I agree -- that's what we did in in linuxbios 1999-2006) and so we'll
probably pursue that path instead. But it's good to know that this is
possible.

For more, see https://github.com/rminnich/linux/tree/monitor

The test is simple, outb to 0xb2 (IIRC) and you'll see the SMI handler in
the kernel print something.
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