diff --git a/lib/libsysdecode/Makefile b/lib/libsysdecode/Makefile
--- a/lib/libsysdecode/Makefile
+++ b/lib/libsysdecode/Makefile
@@ -121,6 +121,11 @@
 CFLAGS+=-DPF
 .endif
 
+ZFSINC=	${SRCTOP}/sys/contrib/openzfs/include
+.if ${MK_ZFS} != "no"
+CFLAGS+=-DZFS -I${ZFSINC}
+.endif
+
 # Workaround duplicate declarations in <netinet/ip_compat.h>
 CFLAGS.gcc.ioctl.c+= -Wno-redundant-decls
 
@@ -139,6 +144,7 @@
 .endif
 ioctl.c.tmp: mkioctls .META
 	env CPP="${CPP}" MK_PF="${MK_PF}" \
+	    MK_ZFS="${MK_ZFS}" ZFSINC="${ZFSINC}" \
 	    /bin/sh ${.CURDIR}/mkioctls ${SYSROOT:U${DESTDIR}}${INCLUDEDIR} > ${.TARGET}
 
 ioctl.c: ioctl.c.tmp
diff --git a/lib/libsysdecode/mkioctls b/lib/libsysdecode/mkioctls
--- a/lib/libsysdecode/mkioctls
+++ b/lib/libsysdecode/mkioctls
@@ -75,6 +75,11 @@
 	print "#include <stddef.h>"
 	print "#include <stdint.h>"
 	print "#include <sysdecode.h>"
+	print "#ifdef ZFS"
+	print "#define boolean_t int"
+	print "#define hrtime_t int"
+	print "#include <sys/fs/zfs.h>"
+	print "#endif"
 	print ""
 	print ioctl_includes
 	print ""
@@ -99,9 +104,17 @@
 	printf("if (val == %s)\n", $i);
 	printf("\t\tstr = \"%s\";\n", $i);
 }
-END {
-	print ""
-	print "\treturn (str);"
-	print "}"
-}
 '
+# ZFS is special
+if [ "${MK_ZFS}" = "yes" ]; then
+	sed -nE 's/^	(ZFS_IOC_[A-Z0-9_]+).*$/\1/p' ${ZFSINC}/sys/fs/zfs.h |
+	sort -u | grep -vE 'ZFS_IOC_(FIRST|LAST)' |
+	while read ioc; do
+		printf "\telse if ((val & 0xffff) == $ioc + ('Z' << 8))\n"
+		printf "\t\tstr = \"$ioc\";\n"
+	done
+fi
+
+printf "\n"
+printf "\treturn (str);\n"
+printf "}\n"