amd64: add LASS support

[kib]

Summary

In short, LASS enforces all kernel memory accesses to have bit 63 set to
1, and all userspace accesses have bit 63 set to 0.  Violations of these
rules cause #GP. There are natural exceptions, like SMAP with rflags.AC=1
allows kernel to access userspace.

Enablement is simple, we need to set CR4.LASS bit on all CPUs.  There
are complications when kernel has to execute code at low addresses, e.g.
for la57 trampoline, or calling into EFI RT.  The patch turns CR4.LASS
off around these regions.

LASS is officially documented in SDM, since at least rev. 085, October
2024.  Tested in simics.

[markj]

sys/amd64/amd64/machdep.c

1358

Why here and not in initializecpu(), where similar features like SMAP are enabled?

sys/x86/x86/mp_x86.c

1137 ↗

(On Diff #171645)

Same here, why isn't this handled in initializecpu()?

[kib]

Move setting CR4.LASS into initializecpu()

[kib]

Require SMAP for LASS.

[kib]

Add comment why SMAP is required.

[markj]

sys/x86/x86/mp_x86.c

1135 ↗

(On Diff #171747)

This part of the change can be dropped I think.

[kib]

Drop unneeded chunk.

[emaste]

We'll want to add a note to mitigations(7) as well; I can take a look at that.

sys/amd64/amd64/initcpu.c

294

Let's expand LASS once on first use - Linear Address Space Separation (LASS). Where we fetch the tunable seems as good a place as any.

[kib]

In D55218#1263398, @emaste wrote:

We'll want to add a note to mitigations(7) as well; I can take a look at that.

Note that this is not quite a mitigation. It is more a switch that makes CPU accept the typical, of not only, UVA/KVA layout used on amd64.

But indeed, LASS short-circuits even a possibility for things like meltdown.

[kib]

Expand LASS in the comment.