KVM: arm64: Walk userspace page tables to compute the THP mapping size

We currently rely on the kvm_is_transparent_hugepage() helper to
discover whether a given page has the potential to be mapped as
a block mapping.

However, this API doesn't really give un everything we want:
- we don't get the size: this is not crucial today as we only
  support PMD-sized THPs, but we'd like to have larger sizes
  in the future
- we're the only user left of the API, and there is a will
  to remove it altogether

To address the above, implement a simple walker using the existing
page table infrastructure, and plumb it into transparent_hugepage_adjust().
No new page sizes are supported in the process.
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Link: https://lore.kernel.org/r/20210726153552.1535838-3-maz@kernel.org

arch/arm64/kvm/mmu.c

@@ -433,6 +433,32 @@ int create_hyp_exec_mappings(phys_addr_t phys_addr, size_t size,
 	return 0;
 }
 
+static struct kvm_pgtable_mm_ops kvm_user_mm_ops = {
+	/* We shouldn't need any other callback to walk the PT */
+	.phys_to_virt		= kvm_host_va,
+};
+
+static int get_user_mapping_size(struct kvm *kvm, u64 addr)
+{
+	struct kvm_pgtable pgt = {
+		.pgd		= (kvm_pte_t *)kvm->mm->pgd,
+		.ia_bits	= VA_BITS,
+		.start_level	= (KVM_PGTABLE_MAX_LEVELS -
+				   CONFIG_PGTABLE_LEVELS),
+		.mm_ops		= &kvm_user_mm_ops,
+	};
+	kvm_pte_t pte = 0;	/* Keep GCC quiet... */
+	u32 level = ~0;
+	int ret;
+
+	ret = kvm_pgtable_get_leaf(&pgt, addr, &pte, &level);
+	VM_BUG_ON(ret);
+	VM_BUG_ON(level >= KVM_PGTABLE_MAX_LEVELS);
+	VM_BUG_ON(!(pte & PTE_VALID));
+
+	return BIT(ARM64_HW_PGTABLE_LEVEL_SHIFT(level));
+}
+
 static struct kvm_pgtable_mm_ops kvm_s2_mm_ops = {
 	.zalloc_page		= stage2_memcache_zalloc_page,
 	.zalloc_pages_exact	= kvm_host_zalloc_pages_exact,
@@ -780,7 +806,7 @@ static bool fault_supports_stage2_huge_mapping(struct kvm_memory_slot *memslot,
  * Returns the size of the mapping.
  */
 static unsigned long
-transparent_hugepage_adjust(struct kvm_memory_slot *memslot,
+transparent_hugepage_adjust(struct kvm *kvm, struct kvm_memory_slot *memslot,
 			    unsigned long hva, kvm_pfn_t *pfnp,
 			    phys_addr_t *ipap)
 {
@@ -791,8 +817,8 @@ transparent_hugepage_adjust(struct kvm_memory_slot *memslot,
 	 * sure that the HVA and IPA are sufficiently aligned and that the
 	 * block map is contained within the memslot.
 	 */
-	if (kvm_is_transparent_hugepage(pfn) &&
-	    fault_supports_stage2_huge_mapping(memslot, hva, PMD_SIZE)) {
+	if (fault_supports_stage2_huge_mapping(memslot, hva, PMD_SIZE) &&
+	    get_user_mapping_size(kvm, hva) >= PMD_SIZE) {
 		/*
 		 * The address we faulted on is backed by a transparent huge
 		 * page.  However, because we map the compound huge page and
@@ -1051,7 +1077,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	 * backed by a THP and thus use block mapping if possible.
 	 */
 	if (vma_pagesize == PAGE_SIZE && !(force_pte || device))
-		vma_pagesize = transparent_hugepage_adjust(memslot, hva,
+		vma_pagesize = transparent_hugepage_adjust(kvm, memslot, hva,
 							   &pfn, &fault_ipa);
 
 	if (fault_status != FSC_PERM && !device && kvm_has_mte(kvm)) {