KVM: arm64: Generate hyp_constants.h for the host

In order to avoid exposing hypervisor (EL2) data structures directly to
the host, generate hyp_constants.h to provide constants such as structure
sizes to the host without dragging in the definitions themselves.
Signed-off-by: Will Deacon <will@kernel.org>
Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211202171048.26924-3-will@kernel.org

arch/arm64/kvm/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
+hyp_constants.h

arch/arm64/kvm/Makefile

@@ -25,3 +25,19 @@ kvm-y := $(KVM)/kvm_main.o $(KVM)/coalesced_mmio.o $(KVM)/eventfd.o \
 	 vgic/vgic-its.o vgic/vgic-debug.o
 
 kvm-$(CONFIG_HW_PERF_EVENTS)  += pmu-emul.o
+
+always-y := hyp_constants.h hyp-constants.s
+
+define rule_gen_hyp_constants
+	$(call filechk,offsets,__HYP_CONSTANTS_H__)
+endef
+
+CFLAGS_hyp-constants.o = -I $(srctree)/$(src)/hyp/include
+$(obj)/hyp-constants.s: $(src)/hyp/hyp-constants.c FORCE
+	$(call if_changed_dep,cc_s_c)
+
+$(obj)/hyp_constants.h: $(obj)/hyp-constants.s FORCE
+	$(call if_changed_rule,gen_hyp_constants)
+
+obj-kvm := $(addprefix $(obj)/, $(kvm-y))
+$(obj-kvm): $(obj)/hyp_constants.h

arch/arm64/kvm/hyp/hyp-constants.c

+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <linux/kbuild.h>
+#include <nvhe/memory.h>
+
+int main(void)
+{
+	DEFINE(STRUCT_HYP_PAGE_SIZE,	sizeof(struct hyp_page));
+	return 0;
+}