KVM: selftests: Split out helper to allocate guest mem via memfd

Extract the code for allocating guest memory via memfd out of
vm_userspace_mem_region_add() and into a new helper, kvm_memfd_alloc().
A future selftest to populate a guest with the maximum amount of guest
memory will abuse KVM's memslots to alias guest memory regions to a
single memfd-backed host region, i.e. needs to back a guest with memfd
memory without a 1:1 association between a memslot and a memfd instance.

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220226001546.360188-27-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/include/kvm_util_base.h

@@ -123,6 +123,7 @@ int kvm_memcmp_hva_gva(void *hva, struct kvm_vm *vm, const vm_vaddr_t gva,
 		       size_t len);
 
 void kvm_vm_elf_load(struct kvm_vm *vm, const char *filename);
+int kvm_memfd_alloc(size_t size, bool hugepages);
 
 void vm_dump(FILE *stream, struct kvm_vm *vm, uint8_t indent);
 

tools/testing/selftests/kvm/lib/kvm_util.c

@@ -718,6 +718,27 @@ void kvm_vm_free(struct kvm_vm *vmp)
 	free(vmp);
 }
 
+int kvm_memfd_alloc(size_t size, bool hugepages)
+{
+	int memfd_flags = MFD_CLOEXEC;
+	int fd, r;
+
+	if (hugepages)
+		memfd_flags |= MFD_HUGETLB;
+
+	fd = memfd_create("kvm_selftest", memfd_flags);
+	TEST_ASSERT(fd != -1, "memfd_create() failed, errno: %i (%s)",
+		    errno, strerror(errno));
+
+	r = ftruncate(fd, size);
+	TEST_ASSERT(!r, "ftruncate() failed, errno: %i (%s)", errno, strerror(errno));
+
+	r = fallocate(fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, 0, size);
+	TEST_ASSERT(!r, "fallocate() failed, errno: %i (%s)", errno, strerror(errno));
+
+	return fd;
+}
+
 /*
  * Memory Compare, host virtual to guest virtual
  *
@@ -970,24 +991,9 @@ void vm_userspace_mem_region_add(struct kvm_vm *vm,
 		region->mmap_size += alignment;
 
 	region->fd = -1;
-	if (backing_src_is_shared(src_type)) {
-		int memfd_flags = MFD_CLOEXEC;
-
-		if (src_type == VM_MEM_SRC_SHARED_HUGETLB)
-			memfd_flags |= MFD_HUGETLB;
-
-		region->fd = memfd_create("kvm_selftest", memfd_flags);
-		TEST_ASSERT(region->fd != -1,
-			    "memfd_create failed, errno: %i", errno);
-
-		ret = ftruncate(region->fd, region->mmap_size);
-		TEST_ASSERT(ret == 0, "ftruncate failed, errno: %i", errno);
-
-		ret = fallocate(region->fd,
-				FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, 0,
-				region->mmap_size);
-		TEST_ASSERT(ret == 0, "fallocate failed, errno: %i", errno);
-	}
+	if (backing_src_is_shared(src_type))
+		region->fd = kvm_memfd_alloc(region->mmap_size,
+					     src_type == VM_MEM_SRC_SHARED_HUGETLB);
 
 	region->mmap_start = mmap(NULL, region->mmap_size,
 				  PROT_READ | PROT_WRITE,