diff --git a/vmm/src/userfaultfd.rs b/vmm/src/userfaultfd.rs
new file mode 100644
index 000000000..3f8447a32
--- /dev/null
+++ b/vmm/src/userfaultfd.rs
@@ -0,0 +1,25 @@
+// Copyright © 2026 Cloud Hypervisor Authors
+//
+// SPDX-License-Identifier: Apache-2.0
+
+// See include/uapi/linux/userfaultfd.h in the kernel code.
+pub const UFFDIO_API: u64 = 0xc018_aa3f; // _IOWR(0xAA, 0x3F, struct uffdio_api)
+pub const UFFDIO_REGISTER: u64 = 0xc020_aa00; // _IOWR(0xAA, 0x00, struct uffdio_register)
+pub const UFFDIO_COPY: u64 = 0xc028_aa03; // _IOWR(0xAA, 0x03, struct uffdio_copy)
+pub const UFFDIO_WAKE: u64 = 0x4010_aa02; // _IOW(0xAA, 0x02, struct uffdio_range)
+
+// Seccomp compares these as Dword (u32); ensure they fit.
+const _: () = assert!(UFFDIO_API <= u32::MAX as u64);
+const _: () = assert!(UFFDIO_REGISTER <= u32::MAX as u64);
+const _: () = assert!(UFFDIO_COPY <= u32::MAX as u64);
+const _: () = assert!(UFFDIO_WAKE <= u32::MAX as u64);
+
+pub const UFFD_API: u64 = 0xAA;
+pub const UFFDIO_REGISTER_MODE_MISSING: u64 = 1;
+pub const UFFD_EVENT_PAGEFAULT: u8 = 0x12;
+pub const UFFD_FEATURE_MISSING_SHMEM: u64 = 1 << 5;
+pub const UFFD_FEATURE_MISSING_HUGETLBFS: u64 = 1 << 6;
+
+const _UFFDIO_COPY: u64 = 0x03;
+const _UFFDIO_WAKE: u64 = 0x02;
+pub const UFFD_API_RANGE_IOCTLS_BASIC: u64 = (1 << _UFFDIO_WAKE) | (1 << _UFFDIO_COPY);