KVM: selftests: Add test coverage for guest_memfd without GUEST_MEMFD_FLAG_MMAP

If a VM type supports KVM_CAP_GUEST_MEMFD_MMAP, the guest_memfd test will
run all test cases with GUEST_MEMFD_FLAG_MMAP set.  This leaves the code
path for creating a non-mmap()-able guest_memfd on a VM that supports
mappable guest memfds untested.

Refactor the test to run the main test suite with a given set of flags.
Then, for VM types that support the mappable capability, invoke the test
suite twice: once with no flags, and once with GUEST_MEMFD_FLAG_MMAP
set.

This ensures both creation paths are properly exercised on capable VMs.

Run test_guest_memfd_flags() only once per VM type since it depends only
on the set of valid/supported flags, i.e. iterating over an arbitrary set
of flags is both unnecessary and wrong.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
[sean: use double-underscores for the inner helper]
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20251003232606.4070510-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: Ackerley Tng

tools/testing/selftests/kvm/guest_memfd_test.c

@@ -268,18 +268,8 @@ do {									\
 	close(fd);							\
 } while (0)
 
-static void test_guest_memfd(unsigned long vm_type)
+static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags)
 {
-	struct kvm_vm *vm;
-	uint64_t flags;
-
-	vm = vm_create_barebones_type(vm_type);
-	flags = vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS);
-
-	/* This test doesn't yet support testing mmap() on private memory. */
-	if (!(flags & GUEST_MEMFD_FLAG_INIT_SHARED))
-		flags &= ~GUEST_MEMFD_FLAG_MMAP;
-
 	test_create_guest_memfd_multiple(vm);
 	test_create_guest_memfd_invalid_sizes(vm, flags);
 
@@ -295,9 +285,24 @@ static void test_guest_memfd(unsigned long vm_type)
 	gmem_test(file_size, vm, flags);
 	gmem_test(fallocate, vm, flags);
 	gmem_test(invalid_punch_hole, vm, flags);
+}
+
+static void test_guest_memfd(unsigned long vm_type)
+{
+	struct kvm_vm *vm = vm_create_barebones_type(vm_type);
+	uint64_t flags;
 
 	test_guest_memfd_flags(vm);
 
+	__test_guest_memfd(vm, 0);
+
+	flags = vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS);
+
+	/* MMAP should always be supported if INIT_SHARED is supported. */
+	if (flags & GUEST_MEMFD_FLAG_INIT_SHARED)
+		__test_guest_memfd(vm, GUEST_MEMFD_FLAG_MMAP |
+				       GUEST_MEMFD_FLAG_INIT_SHARED);
+
 	kvm_vm_free(vm);
 }