Add support for Privileged Access Never (PAN)

[andrew]

Summary

In ARMv8.1 ARM has added a process state bit to disable access to userspace
from the kernel. Make use of this to restrict accessing userspace to just
the functions that explicitly handle crossing the user kernel boundary.

Test Plan

Booted on ThunderX Pass 1.1 (no PAN), and 2.0 (with PAN).

[andrew]

Fix a typo

[kib]

Do we need to set SCTLR_EL1.SPAN to 1 ?

[andrew]

In D10371#214953, @kib wrote:

Do we need to set SCTLR_EL1.SPAN to 1 ?

No, we need to clear it so PSTATE.PAN is set to 1 on exception entry.

[andrew]

Clear SCTLR_EL1.SPAN

[kib]

In D10371#215086, @andrew wrote:

In D10371#214953, @kib wrote:

Do we need to set SCTLR_EL1.SPAN to 1 ?

No, we need to clear it so PSTATE.PAN is set to 1 on exception entry.

Right, clear, not set.