diff --git a/share/man/man4/pci.4 b/share/man/man4/pci.4
--- a/share/man/man4/pci.4
+++ b/share/man/man4/pci.4
@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd August 20, 2025
+.Dd August 31, 2025
 .Dt PCI 4
 .Os
 .Sh NAME
@@ -237,6 +237,15 @@
 Driver unit number.
 .It pd_numa_domain
 Driver NUMA domain.
+.It pc_reported_len
+Length of the valid portion of the encompassing
+.Vt pci_conf
+structure.
+This should always be equivalent to the offset of the
+.Va pc_spare
+member.
+.It pc_spare
+Reserved for future use.
 .El
 .It offset
 The offset is passed in by the user to tell the kernel where it should
diff --git a/sys/dev/pci/pci_user.c b/sys/dev/pci/pci_user.c
--- a/sys/dev/pci/pci_user.c
+++ b/sys/dev/pci/pci_user.c
@@ -80,6 +80,8 @@
 	char		pd_name[PCI_MAXNAMELEN + 1];  /* device name */
 	u_int32_t	pd_unit;	/* device unit number */
 	int		pd_numa_domain;	/* device NUMA domain */
+	u_int32_t	pc_reported_len;/* length of PCI data reported */
+	char		pc_spare[64];	/* space for future fields */
 };
 
 struct pci_match_conf32 {
@@ -885,8 +887,11 @@
 		strlcpy(pcup->pc32.pd_name, pcp->pd_name,
 		    sizeof(pcup->pc32.pd_name));
 		pcup->pc32.pd_unit = (uint32_t)pcp->pd_unit;
-		if (cmd == PCIOCGETCONF32)
+		if (cmd == PCIOCGETCONF32) {
 			pcup->pc32.pd_numa_domain = pcp->pd_numa_domain;
+			pcup->pc32.pc_reported_len =
+			    (uint32_t)offsetof(struct pci_conf32, pc_spare);
+		}
 		return;
 #endif /* COMPAT_FREEBSD32 */
 
@@ -1326,6 +1331,9 @@
 					break;
 				}
 
+				dinfo->conf.pc_reported_len =
+				    offsetof(struct pci_conf, pc_spare);
+
 				pci_conf_for_copyout(&dinfo->conf, &pcu, cmd);
 				error = copyout(&pcu,
 				    (caddr_t)cio->matches +
diff --git a/sys/sys/pciio.h b/sys/sys/pciio.h
--- a/sys/sys/pciio.h
+++ b/sys/sys/pciio.h
@@ -78,6 +78,8 @@
 	char		pd_name[PCI_MAXNAMELEN + 1];  /* device name */
 	u_long		pd_unit;	/* device unit number */
 	int		pd_numa_domain;	/* device NUMA domain */
+	size_t		pc_reported_len;/* length of PCI data reported */
+	char		pc_spare[64];	/* space for future fields */
 };
 
 struct pci_match_conf {