KVM: arm/arm64: Properly handle faulting of device mappings

A device mapping is normally always mapped at Stage-2, since there
is very little gain in having it faulted in.

Nonetheless, it is possible to end-up in a situation where the device
mapping has been removed from Stage-2 (userspace munmaped the VFIO
region, and the MMU notifier did its job), but present in a userspace
mapping (userpace has mapped it back at the same address). In such
a situation, the device mapping will be demand-paged as the guest
performs memory accesses.

This requires to be careful when dealing with mapping size, cache
management, and to handle potential execution of a device mapping.

Reported-by: Alexandru Elisei <[email protected]>
Signed-off-by: Marc Zyngier <[email protected]>
Tested-by: Alexandru Elisei <[email protected]>
Reviewed-by: James Morse <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]

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