arm/arm64: KVM: Add PSCI version selection API

Although we've implemented PSCI 0.1, 0.2 and 1.0, we expose either 0.1
or 1.0 to a guest, defaulting to the latest version of the PSCI
implementation that is compatible with the requested version. This is
no different from doing a firmware upgrade on KVM.

But in order to give a chance to hypothetical badly implemented guests
that would have a fit by discovering something other than PSCI 0.2,
let's provide a new API that allows userspace to pick one particular
version of the API.

This is implemented as a new class of "firmware" registers, where
we expose the PSCI version. This allows the PSCI version to be
save/restored as part of a guest migration, and also set to
any supported version if the guest requires it.

Cc: stable@vger.kernel.org #4.16
Reviewed-by: Christoffer Dall <cdall@kernel.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

arch/arm64/include/asm/kvm_host.h

@@ -75,6 +75,9 @@ struct kvm_arch {
 
 	/* Interrupt controller */
 	struct vgic_dist	vgic;
+
+	/* Mandated version of PSCI */
+	u32 psci_version;
 };
 
 #define KVM_NR_MEM_OBJS     40