This is defined in linux/Documentation/arm64/booting.txt.
cb5e832 xen: arm: support for loading 64-bit zImage dom0
xen/arch/arm/kernel.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 76 insertions(+), 5 deletions(-)
Upstream: xenbits.xen.org