kvm: arm/arm64: Force reading uncached stage2 PGD

commit 2952a607 upstream.

Make sure we don't use a cached value of the KVM stage2 PGD while
resetting the PGD.

Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: Christoffer Dall <cdall@linaro.org>
Signed-off-by: Christoffer Dall <cdall@linaro.org>
[bwh: Backported to 3.16:
 - s/READ_ONCE/ACCESS_ONCE/
 - Adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/arm/kvm/mmu.c

@@ -643,7 +643,7 @@ void kvm_free_stage2_pgd(struct kvm *kvm)
 	spin_lock(&kvm->mmu_lock);
 	if (kvm->arch.pgd) {
 		unmap_stage2_range(kvm, 0, KVM_PHYS_SIZE);
-		pgd = kvm->arch.pgd;
+		pgd = ACCESS_ONCE(kvm->arch.pgd);
 		kvm->arch.pgd = NULL;
 	}
 	spin_unlock(&kvm->mmu_lock);