KVM: arm64: nv: Handle shadow stage 2 page faults

If we are faulting on a shadow stage 2 translation, we first walk the
guest hypervisor's stage 2 page table to see if it has a mapping. If
not, we inject a stage 2 page fault to the virtual EL2. Otherwise, we
create a mapping in the shadow stage 2 page table.

Note that we have to deal with two IPAs when we got a shadow stage 2
page fault. One is the address we faulted on, and is in the L2 guest
phys space. The other is from the guest stage-2 page table walk, and is
in the L1 guest phys space.  To differentiate them, we rename variables
so that fault_ipa is used for the former and ipa is used for the latter.

When mapping a page in a shadow stage-2, special care must be taken not
to be more permissive than the guest is.

Co-developed-by: Christoffer Dall <[email protected]>
Co-developed-by: Jintack Lim <[email protected]>
Signed-off-by: Christoffer Dall <[email protected]>
Signed-off-by: Jintack Lim <[email protected]>
Signed-off-by: Marc Zyngier <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Oliver Upton <[email protected]>

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