The hcall H_ENTER_NESTED takes two parameters: the address in L1 guest
memory of a hv_regs struct and the address of a pt_regs struct.  The
hcall requests the L0 hypervisor to use the register values in these
structs to run a L2 guest and to return the exit state of the L2 guest
in these structs.  These are in the endianness of the L1 guest, rather
than being always big-endian as is usually the case for PAPR
hypercalls.

This is convenient because it means that the L1 guest can pass the
address of the regs field in its kvm_vcpu_arch struct.  This also
improves performance slightly by avoiding the need for two copies of
the pt_regs struct.

When reading/writing these structures, this patch handles the case
where the endianness of the L1 guest differs from that of the L0
hypervisor, by byteswapping the structures after reading and before
writing them back.

Since all the fields of the pt_regs are of the same type, i.e.,
unsigned long, we treat it as an array of unsigned longs.  The fields
of struct hv_guest_state are not all the same, so its fields are
byteswapped individually.
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>