* Internal ABI between the kernel and HYP

This file documents the interaction between the Linux kernel and the
hypervisor layer when running Linux as a hypervisor (for example
KVM). It doesn't cover the interaction of the kernel with the
hypervisor when running as a guest (under Xen, KVM or any other
hypervisor), or any hypervisor-specific interaction when the kernel is
used as a host.

On arm and arm64 (without VHE), the kernel doesn't run in hypervisor
mode, but still needs to interact with it, allowing a built-in
hypervisor to be either installed or torn down.

In order to achieve this, the kernel must be booted at HYP (arm) or
EL2 (arm64), allowing it to install a set of stubs before dropping to
SVC/EL1. These stubs are accessible by using a 'hvc #0' instruction,
and only act on individual CPUs.

Unless specified otherwise, any built-in hypervisor must implement
these functions (see arch/arm{,64}/include/asm/virt.h):

* r0/x0 = HVC_SET_VECTORS
  r1/x1 = vectors

  Set HVBAR/VBAR_EL2 to 'vectors' to enable a hypervisor. 'vectors'
  must be a physical address, and respect the alignment requirements
  of the architecture. Only implemented by the initial stubs, not by
  Linux hypervisors.

* r0/x0 = HVC_RESET_VECTORS

  Turn HYP/EL2 MMU off, and reset HVBAR/VBAR_EL2 to the initials
  stubs' exception vector value. This effectively disables an existing
  hypervisor.

* r0/x0 = HVC_SOFT_RESTART
  r1/x1 = restart address
  x2 = x0's value when entering the next payload (arm64)
  x3 = x1's value when entering the next payload (arm64)
  x4 = x2's value when entering the next payload (arm64)

  Mask all exceptions, disable the MMU, move the arguments into place
  (arm64 only), and jump to the restart address while at HYP/EL2. This
  hypercall is not expected to return to its caller.

Any other value of r0/x0 triggers a hypervisor-specific handling,
which is not documented here.

The return value of a stub hypercall is held by r0/x0, and is 0 on
success, and HVC_STUB_ERR on error. A stub hypercall is allowed to
clobber any of the caller-saved registers (x0-x18 on arm64, r0-r3 and
ip on arm). It is thus recommended to use a function call to perform
the hypercall.