Put the arm64 vttbr_el2 register into a state

Zero the vttbr_el2 register on each CPU so we can tell if we are
running the host or guest kernel from a hypervisor.

Obtained from:	https://github.com/FreeBSD-UPB/freebsd-src (earlier version)
Sponsored by:	Innovate UK
Sponsored by:	The FreeBSD Foundation

sys/arm64/arm64/locore.S

@@ -295,6 +295,9 @@ LENTRY(drop_to_el1)
 	add	x2, x2, :lo12:hyp_vectors
 	msr	vbar_el2, x2
 
+	/* Zero vttbr_el2 so a hypervisor can tell the host and guest apart */
+	msr	vttbr_el2, xzr
+
 	mov	x2, #(PSR_F | PSR_I | PSR_A | PSR_D | PSR_M_EL1h)
 	msr	spsr_el2, x2
 

sys/arm64/include/hypervisor.h

@@ -199,6 +199,7 @@
 /* VTTBR_EL2 - Virtualization Translation Table Base Register */
 #define	VTTBR_VMID_MASK		0xffff000000000000
 #define	VTTBR_VMID_SHIFT	48
+/* Assumed to be 0 by locore.S */
 #define	VTTBR_HOST		0x0000000000000000
 
 #endif /* !_MACHINE_HYPERVISOR_H_ */