KVM: selftests: Use a loop to create guest page tables

Walk the guest page tables via a loop when creating new mappings,
instead of using unique variables for each level of the page tables.

This simplifies the code and makes it easier to support 5-level paging
in the future.

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251028225827.2269128-2-jmattson@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: jsmattsonjr

tools/testing/selftests/kvm/lib/x86/processor.c

@@ -218,8 +218,8 @@ static uint64_t *virt_create_upper_pte(struct kvm_vm *vm,
 void __virt_pg_map(struct kvm_vm *vm, uint64_t vaddr, uint64_t paddr, int level)
 {
 	const uint64_t pg_size = PG_LEVEL_SIZE(level);
-	uint64_t *pml4e, *pdpe, *pde;
-	uint64_t *pte;
+	uint64_t *pte = &vm->pgd;
+	int current_level;
 
 	TEST_ASSERT(vm->mode == VM_MODE_PXXV48_4K,
 		    "Unknown or unsupported guest mode, mode: 0x%x", vm->mode);
@@ -243,20 +243,17 @@ void __virt_pg_map(struct kvm_vm *vm, uint64_t vaddr, uint64_t paddr, int level)
 	 * Allocate upper level page tables, if not already present.  Return
 	 * early if a hugepage was created.
 	 */
-	pml4e = virt_create_upper_pte(vm, &vm->pgd, vaddr, paddr, PG_LEVEL_512G, level);
-	if (*pml4e & PTE_LARGE_MASK)
-		return;
-
-	pdpe = virt_create_upper_pte(vm, pml4e, vaddr, paddr, PG_LEVEL_1G, level);
-	if (*pdpe & PTE_LARGE_MASK)
-		return;
-
-	pde = virt_create_upper_pte(vm, pdpe, vaddr, paddr, PG_LEVEL_2M, level);
-	if (*pde & PTE_LARGE_MASK)
-		return;
+	for (current_level = vm->pgtable_levels;
+	     current_level > PG_LEVEL_4K;
+	     current_level--) {
+		pte = virt_create_upper_pte(vm, pte, vaddr, paddr,
+					    current_level, level);
+		if (*pte & PTE_LARGE_MASK)
+			return;
+	}
 
 	/* Fill in page table entry. */
-	pte = virt_get_pte(vm, pde, vaddr, PG_LEVEL_4K);
+	pte = virt_get_pte(vm, pte, vaddr, PG_LEVEL_4K);
 	TEST_ASSERT(!(*pte & PTE_PRESENT_MASK),
 		    "PTE already present for 4k page at vaddr: 0x%lx", vaddr);
 	*pte = PTE_PRESENT_MASK | PTE_WRITABLE_MASK | (paddr & PHYSICAL_PAGE_MASK);