MIPS: KVM: Restore host EBase from ebase variable

The host kernel's exception vector base address is currently saved in
the VCPU structure at creation time, and restored on a guest exit.
However it doesn't change and can already be easily accessed from the
'ebase' variable (arch/mips/kernel/traps.c), so drop the host_ebase
member of kvm_vcpu_arch, export the 'ebase' variable to modules and load
from there instead.

This does result in a single extra instruction (lui) on the guest exit
path, but simplifies the code a bit and removes the redundant storage of
the host exception base address.

Credit for the idea goes to Cavium's VZ KVM implementation.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/mips/kvm/mips.c

@@ -273,9 +273,6 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm, unsigned int id)
 	else
 		size = 0x4000;
 
-	/* Save Linux EBASE */
-	vcpu->arch.host_ebase = (void *)read_c0_ebase();
-
 	gebase = kzalloc(ALIGN(size, PAGE_SIZE), GFP_KERNEL);
 
 	if (!gebase) {