arm/arm64: KVM: make the value of ICC_SRE_EL1 a per-VM variable

ICC_SRE_EL1 is a system register allowing msr/mrs accesses to the
GIC CPU interface for EL1 (guests). Currently we force it to 0, but
for proper GICv3 support we have to allow guests to use it (depending
on their selected virtual GIC model).
So add ICC_SRE_EL1 to the list of saved/restored registers on a
world switch, but actually disallow a guest to change it by only
restoring a fixed, once-initialized value.
This value depends on the GIC model userland has chosen for a guest.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
index 1c0e9db..ff04afd 100644
--- a/include/kvm/arm_vgic.h
+++ b/include/kvm/arm_vgic.h
@@ -245,6 +245,7 @@
 #ifdef CONFIG_ARM_GIC_V3
 	u32		vgic_hcr;
 	u32		vgic_vmcr;
+	u32		vgic_sre;	/* Restored only, change ignored */
 	u32		vgic_misr;	/* Saved only */
 	u32		vgic_eisr;	/* Saved only */
 	u32		vgic_elrsr;	/* Saved only */