KVM: arm64: Introduce KVM_PGTABLE_S2_IDMAP stage 2 flag

Introduce a new stage 2 configuration flag to specify that all mappings
in a given page-table will be identity-mapped, as will be the case for
the host. This allows to introduce sanity checks in the map path and to
avoid programming errors.
Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210319100146.1149909-34-qperret@google.com

arch/arm64/include/asm/kvm_pgtable.h

@@ -60,9 +60,11 @@ struct kvm_pgtable_mm_ops {
  * enum kvm_pgtable_stage2_flags - Stage-2 page-table flags.
  * @KVM_PGTABLE_S2_NOFWB:	Don't enforce Normal-WB even if the CPUs have
  *				ARM64_HAS_STAGE2_FWB.
+ * @KVM_PGTABLE_S2_IDMAP:	Only use identity mappings.
  */
 enum kvm_pgtable_stage2_flags {
 	KVM_PGTABLE_S2_NOFWB			= BIT(0),
+	KVM_PGTABLE_S2_IDMAP			= BIT(1),
 };
 
 /**

arch/arm64/kvm/hyp/pgtable.c

@@ -760,6 +760,9 @@ int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
 		.arg		= &map_data,
 	};
 
+	if (WARN_ON((pgt->flags & KVM_PGTABLE_S2_IDMAP) && (addr != phys)))
+		return -EINVAL;
+
 	ret = stage2_set_prot_attr(pgt, prot, &map_data.attr);
 	if (ret)
 		return ret;