KVM: arm64: Use fallback mapping sizes for contiguous huge page sizes

Although huge pages can be created out of multiple contiguous PMDs
or PTEs, the corresponding sizes are not supported at Stage-2 yet.

Instead of failing the mapping, fall back to the nearer supported
mapping size (CONT_PMD to PMD and CONT_PTE to PTE respectively).
Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Gavin Shan <gshan@redhat.com>
[maz: rewritten commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201025230626.18501-1-gshan@redhat.com

arch/arm64/kvm/mmu.c

@@ -787,14 +787,26 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 		vma_shift = PAGE_SHIFT;
 	}
 
-	if (vma_shift == PUD_SHIFT &&
-	    !fault_supports_stage2_huge_mapping(memslot, hva, PUD_SIZE))
-	       vma_shift = PMD_SHIFT;
-
-	if (vma_shift == PMD_SHIFT &&
-	    !fault_supports_stage2_huge_mapping(memslot, hva, PMD_SIZE)) {
-		force_pte = true;
+	switch (vma_shift) {
+	case PUD_SHIFT:
+		if (fault_supports_stage2_huge_mapping(memslot, hva, PUD_SIZE))
+			break;
+		fallthrough;
+	case CONT_PMD_SHIFT:
+		vma_shift = PMD_SHIFT;
+		fallthrough;
+	case PMD_SHIFT:
+		if (fault_supports_stage2_huge_mapping(memslot, hva, PMD_SIZE))
+			break;
+		fallthrough;
+	case CONT_PTE_SHIFT:
 		vma_shift = PAGE_SHIFT;
+		force_pte = true;
+		fallthrough;
+	case PAGE_SHIFT:
+		break;
+	default:
+		WARN_ONCE(1, "Unknown vma_shift %d", vma_shift);
 	}
 
 	vma_pagesize = 1UL << vma_shift;