diff --git a/Makefile.inc1 b/Makefile.inc1
index efa1299b76a7e8..99d86cb5a13b1c 100644
--- a/Makefile.inc1
+++ b/Makefile.inc1
@@ -1604,7 +1604,8 @@ _sysent_dirs+=	sys/compat/freebsd32
 _sysent_dirs+=	sys/amd64/linux		\
 		sys/amd64/linux32	\
 		sys/arm64/linux		\
-		sys/i386/linux
+		sys/i386/linux		\
+		sys/tools/syscalls/test
 
 sysent: .PHONY
 .for _dir in ${_sysent_dirs}
diff --git a/sys/kern/syscalls.master b/sys/kern/syscalls.master
index d3c4f2c6423132..7551bd45f83964 100644
--- a/sys/kern/syscalls.master
+++ b/sys/kern/syscalls.master
@@ -107,9 +107,11 @@
 ;	timet_	Object contains a time_t and varies between i386 and other
 ;		ABIs.
 
-; #include's, #defines's, etc. may be included, and are copied to the output
-; files. However, #ifdef, etc will be copied, but any lines that don't start
-; with # will not. Caveat Emptor.
+; #include's, #defines's, etc. may be included, and are copied to a
+; limited set of output files.  Before the first syscalls, #include lines will
+; be copied and %%ABI_HEADERS%% expanded.  Between system call entries,
+; all lines beginning with # will be copied.  Caveat Emptor.
+; WARNING: this functionality is deprecated.
 
 #include <sys/param.h>
 #include <sys/sysent.h>
diff --git a/sys/tools/syscalls/core/freebsd-syscall.lua b/sys/tools/syscalls/core/freebsd-syscall.lua
index 193b1e43563c30..2ef0fdea640123 100644
--- a/sys/tools/syscalls/core/freebsd-syscall.lua
+++ b/sys/tools/syscalls/core/freebsd-syscall.lua
@@ -48,7 +48,9 @@ function FreeBSDSyscall:parseSysfile()
 	end
 
 	local incs = ""
-	local defs = ""
+	local prolog = ""
+	local first = true
+	local cpp_warned = false
 	local s
 	for line in fh:lines() do
 		line = line:gsub(commentExpr, "") -- Strip any comments.
@@ -80,12 +82,23 @@ function FreeBSDSyscall:parseSysfile()
 			if h ~= nil and h ~= "" then
 				incs = incs .. h .. "\n"
 			end
-		elseif line:match("^#%s*define") then
-			defs = defs .. line.. "\n"
 		elseif line:match("^#") then
-			util.abort(1, "Unsupported cpp op " .. line)
+			if not cpp_warned then
+				util.warn("use of non-include cpp " ..
+				    "directives is deprecated")
+				cpp_warned = true
+			end
+			prolog = prolog .. line .. "\n"
 		else
 			s = syscall:new()
+			if first then
+				self.prolog = prolog
+				s.prolog = ""
+				first = false
+			else
+				s.prolog = prolog
+			end
+			prolog = ""
 			if s:add(line) then
 				-- Append to system call list.
 				for t in s:iter() do
@@ -114,7 +127,13 @@ function FreeBSDSyscall:parseSysfile()
 
 	assert(fh:close())
 	self.includes = incs
-	self.defines = defs
+	self.epilog = prolog
+
+	if self.prolog ~= "" then
+		util.warn("non-include pre-processor directives in the " ..
+		    "config prolog will not appear in generated output:\n" ..
+		    self.prolog)
+	end
 end
 
 function FreeBSDSyscall:findStructs()
diff --git a/sys/tools/syscalls/scripts/init_sysent.lua b/sys/tools/syscalls/scripts/init_sysent.lua
index 66683250b482e0..a1f51b5f3152e8 100755
--- a/sys/tools/syscalls/scripts/init_sysent.lua
+++ b/sys/tools/syscalls/scripts/init_sysent.lua
@@ -66,6 +66,8 @@ struct sysent %s[] = {
 		-- based on the type of system call.
 		local comment = v.name
 
+		gen:write(v.prolog);
+
 		-- Handle non-compat:
 		if v:native() then
 			gen:write(string.format(
@@ -163,6 +165,7 @@ struct sysent %s[] = {
 
 		gen:write(string.format("\t/* %d = %s */\n", v.num, comment))
 	end
+	gen:write(tbl.epilog)
 
 	-- End
 	gen:write("};\n")
diff --git a/sys/tools/syscalls/scripts/syscall_mk.lua b/sys/tools/syscalls/scripts/syscall_mk.lua
index 49d3f6f86c201d..8f8755af31c078 100755
--- a/sys/tools/syscalls/scripts/syscall_mk.lua
+++ b/sys/tools/syscalls/scripts/syscall_mk.lua
@@ -43,24 +43,14 @@ function syscall_mk.generate(tbl, config, fh)
 	gen:write("MIASM =  \\\n") -- preamble
 	for _, v in pairs(s) do
 		local c = v:compatLevel()
+		local bs = " \\"
 		idx = idx + 1
-		if v:native() and not v.type.NODEF and not v.type.NOLIB then
+		if (v:native() or c >= 7) and not v.type.NODEF and not v.type.NOLIB then
 			if idx >= size then
 				-- At last system call, no backslash.
-				gen:write(string.format("\t%s.o\n", v:symbol()))
-			else
-				-- Normal behavior.
-				gen:write(string.format("\t%s.o \\\n", v:symbol()))
-			end
-		-- Handle compat (everything >= FREEBSD3):
-		elseif c >= 7 and not v.type.NODEF and not v.type.NOLIB then
-			if idx >= size then
-				-- At last system call, no backslash.
-				gen:write(string.format("\t%s.o\n", v:symbol()))
-			else
-				-- Normal behavior.
-				gen:write(string.format("\t%s.o \\\n", v:symbol()))
+				bs = ""
 			end
+			gen:write(string.format("\t%s.o%s\n", v:symbol(), bs))
 		end
 	end
 end
diff --git a/sys/tools/syscalls/scripts/syscalls.lua b/sys/tools/syscalls/scripts/syscalls.lua
index 38ed396a73aef8..a40ebc1376277d 100755
--- a/sys/tools/syscalls/scripts/syscalls.lua
+++ b/sys/tools/syscalls/scripts/syscalls.lua
@@ -38,6 +38,9 @@ function syscalls.generate(tbl, config, fh)
 	for _, v in pairs(s) do
 		--print("num " .. v.num .. " name " .. v.name)
 		local c = v:compatLevel()
+
+		gen:write(v.prolog);
+
 		if v:native() then
 			gen:write(string.format([[
 	"%s",			/* %d = %s */
@@ -80,6 +83,7 @@ function syscalls.generate(tbl, config, fh)
 
 		end
 	end
+	gen:write(tbl.epilog)
 	-- End
 	gen:write("};\n")
 end
diff --git a/sys/tools/syscalls/scripts/syscalls_map.lua b/sys/tools/syscalls/scripts/syscalls_map.lua
index 023b4305292197..52c3b294e33876 100755
--- a/sys/tools/syscalls/scripts/syscalls_map.lua
+++ b/sys/tools/syscalls/scripts/syscalls_map.lua
@@ -37,7 +37,7 @@ function syscalls_map.generate(tbl, config, fh)
 	gen:write(string.format("FBSDprivate_1.0 {\n"))
 
 	for _, v in pairs(s) do
-		--print("num " .. v.num .. " name " .. v.name)
+		gen:write(v.prolog)
 		if v:native() and not v.type.NODEF and not v.type.NOLIB then
 			if v.name ~= "exit" and v.name ~= "vfork" then
 				gen:write(string.format("\t_%s;\n", v.name))
@@ -45,6 +45,8 @@ function syscalls_map.generate(tbl, config, fh)
 			gen:write(string.format("\t__sys_%s;\n", v.name))
 		end
 	end
+	gen:write(tbl.epilog)
+
 	-- End
 	gen:write("};\n")
 end
diff --git a/sys/tools/syscalls/scripts/sysproto_h.lua b/sys/tools/syscalls/scripts/sysproto_h.lua
index 6770e054889951..2b0b9293ea6678 100755
--- a/sys/tools/syscalls/scripts/sysproto_h.lua
+++ b/sys/tools/syscalls/scripts/sysproto_h.lua
@@ -93,6 +93,12 @@ struct thread;
 		-- intuitive).
 		local audit_idx = 10000 -- this should do
 
+		gen:write(v.prolog)
+		gen:store(v.prolog, 1)
+		for _, w in pairs(config.compat_options) do
+			gen:store(v.prolog, w.compatlevel * 10)
+		end
+
 		-- Handle non-compat:
 		if v:native() then
 			-- All these negation conditions are because (in
@@ -202,6 +208,12 @@ struct %s {
 		    end_idx)
 	end
 
+	gen:write(tbl.epilog)
+	gen:store(tbl.epilog, 1)
+	for _, w in pairs(config.compat_options) do
+		gen:store(tbl.epilog, w.compatlevel * 10)
+	end
+
 	if gen.storage_levels ~= nil then
 		gen:writeStorage()
 	end
diff --git a/sys/tools/syscalls/scripts/systrace_args.lua b/sys/tools/syscalls/scripts/systrace_args.lua
index 88170b85e73737..40dd072bc0c3fd 100755
--- a/sys/tools/syscalls/scripts/systrace_args.lua
+++ b/sys/tools/syscalls/scripts/systrace_args.lua
@@ -68,6 +68,10 @@ systrace_return_setargdesc(int sysnum, int ndx, char *desc, size_t descsz)
 
 	for _, v in pairs(s) do
 
+		gen:write(v.prolog);
+		gen:store(v.prolog, 1);
+		gen:store(v.prolog, 2);
+
 		-- Handle non compat:
 		if v:native() then
 			gen:write(string.format([[
@@ -212,6 +216,7 @@ systrace_return_setargdesc(int sysnum, int ndx, char *desc, size_t descsz)
 		end
 	end
 
+	gen:write(tbl.epilog)
 	gen:write([[
 	default:
 		*n_args = 0;
@@ -219,6 +224,8 @@ systrace_return_setargdesc(int sysnum, int ndx, char *desc, size_t descsz)
 	};
 }
 ]])
+
+	gen:store(tbl.epilog, 1)
 	gen:store([[
 	default:
 		break;
@@ -227,6 +234,8 @@ systrace_return_setargdesc(int sysnum, int ndx, char *desc, size_t descsz)
 		strlcpy(desc, p, descsz);
 }
 ]], 1)
+
+	gen:store(tbl.epilog, 2)
 	gen:store([[
 	default:
 		break;
diff --git a/sys/tools/syscalls/test/Makefile b/sys/tools/syscalls/test/Makefile
new file mode 100644
index 00000000000000..670388ebf52e55
--- /dev/null
+++ b/sys/tools/syscalls/test/Makefile
@@ -0,0 +1,3 @@
+GENERATED_PREFIX=	test_
+
+.include "../../../conf/sysent.mk"
diff --git a/sys/tools/syscalls/test/syscalls.conf b/sys/tools/syscalls/test/syscalls.conf
new file mode 100644
index 00000000000000..ffcfa8d4c6aa65
--- /dev/null
+++ b/sys/tools/syscalls/test/syscalls.conf
@@ -0,0 +1,11 @@
+sysnames="test_syscalls.c"
+sysproto="test_proto.h"
+sysproto_h=_TEST_SYSPROTO_H_
+syshdr="test_syscall.h"
+syssw="test_sysent.c"
+syscallprefix="TEST_SYS_"
+switchname="test_sysent"
+namesname="test_syscallnames"
+systrace="test_systrace_args.c"
+libsysmap="test_syscalls.map"
+compat_set=""
diff --git a/sys/tools/syscalls/test/syscalls.master b/sys/tools/syscalls/test/syscalls.master
new file mode 100644
index 00000000000000..4986ad41fc6a9d
--- /dev/null
+++ b/sys/tools/syscalls/test/syscalls.master
@@ -0,0 +1,26 @@
+#include <sys/param.h>
+#include <sys/sysent.h>
+#include <sys/sysproto.h>
+
+0	AUE_NULL		UNIMPL	unimpl_syscall0
+
+; Scenario #1: Vendor uses a reserved slot on just a single platform
+#ifdef PLATFORM_FOO
+1	AUE_NULL		STD {
+		int syscall1(
+		    int arg1
+		);
+	}
+#else
+1	AUE_NULL		RESERVED
+#endif
+
+; Scenario #2: The other way around; vendor obsoletes a syscall on newer
+; platforms
+#ifdef PLATFORM_FOO
+2	AUE_NULL		OBSOL	syscall2
+#else
+2	AUE_NULL		STD {
+		int syscall2(void);
+	}
+#endif
diff --git a/sys/tools/syscalls/test/test_proto.h b/sys/tools/syscalls/test/test_proto.h
new file mode 100644
index 00000000000000..db1f507b8be093
--- /dev/null
+++ b/sys/tools/syscalls/test/test_proto.h
@@ -0,0 +1,63 @@
+/*
+ * System call prototypes.
+ *
+ * DO NOT EDIT-- this file is automatically @generated.
+ */
+
+#ifndef _TEST_SYSPROTO_H_
+#define	_TEST_SYSPROTO_H_
+
+#include <sys/types.h>
+#include <sys/signal.h>
+#include <sys/cpuset.h>
+#include <sys/domainset.h>
+#include <sys/_ffcounter.h>
+#include <sys/_semaphore.h>
+#include <sys/ucontext.h>
+#include <sys/wait.h>
+
+#include <bsm/audit_kevents.h>
+
+struct proc;
+
+struct thread;
+
+#define	PAD_(t)	(sizeof(syscallarg_t) <= sizeof(t) ? \
+		0 : sizeof(syscallarg_t) - sizeof(t))
+
+#if BYTE_ORDER == LITTLE_ENDIAN
+#define	PADL_(t)	0
+#define	PADR_(t)	PAD_(t)
+#else
+#define	PADL_(t)	PAD_(t)
+#define	PADR_(t)	0
+#endif
+
+#ifdef PLATFORM_FOO
+struct syscall1_args {
+	char arg1_l_[PADL_(int)]; int arg1; char arg1_r_[PADR_(int)];
+};
+#else
+#endif
+#ifdef PLATFORM_FOO
+#else
+struct syscall2_args {
+	syscallarg_t dummy;
+};
+#endif
+#ifdef PLATFORM_FOO
+int	sys_syscall1(struct thread *, struct syscall1_args *);
+#else
+#endif
+#ifdef PLATFORM_FOO
+#else
+int	sys_syscall2(struct thread *, struct syscall2_args *);
+#endif
+#define	TEST_SYS_AUE_syscall1	AUE_NULL
+#define	TEST_SYS_AUE_syscall2	AUE_NULL
+
+#undef PAD_
+#undef PADL_
+#undef PADR_
+
+#endif /* !_TEST_SYSPROTO_H_ */
diff --git a/sys/tools/syscalls/test/test_syscall.h b/sys/tools/syscalls/test/test_syscall.h
new file mode 100644
index 00000000000000..026e82d9ac6271
--- /dev/null
+++ b/sys/tools/syscalls/test/test_syscall.h
@@ -0,0 +1,10 @@
+/*
+ * System call numbers.
+ *
+ * DO NOT EDIT-- this file is automatically @generated.
+ */
+
+#define	TEST_SYS_syscall1	1
+				/* 2 is obsolete syscall2 */
+#define	TEST_SYS_syscall2	2
+#define	TEST_SYS_MAXSYSCALL	3
diff --git a/sys/tools/syscalls/test/test_syscalls.c b/sys/tools/syscalls/test/test_syscalls.c
new file mode 100644
index 00000000000000..0e73c8223507c3
--- /dev/null
+++ b/sys/tools/syscalls/test/test_syscalls.c
@@ -0,0 +1,19 @@
+/*
+ * System call names.
+ *
+ * DO NOT EDIT-- this file is automatically @generated.
+ */
+
+const char *test_syscallnames[] = {
+	"#0",			/* 0 = unimpl_syscall0 */
+#ifdef PLATFORM_FOO
+	"syscall1",			/* 1 = syscall1 */
+#else
+	"#1",			/* 1 = reserved for local use */
+#endif
+#ifdef PLATFORM_FOO
+	"obs_syscall2",			/* 2 = obsolete syscall2 */
+#else
+	"syscall2",			/* 2 = syscall2 */
+#endif
+};
diff --git a/sys/tools/syscalls/test/test_syscalls.map b/sys/tools/syscalls/test/test_syscalls.map
new file mode 100644
index 00000000000000..137496186402de
--- /dev/null
+++ b/sys/tools/syscalls/test/test_syscalls.map
@@ -0,0 +1,18 @@
+/*
+ * FreeBSD system call symbols.
+ *
+ * DO NOT EDIT-- this file is automatically @generated.
+ */
+
+FBSDprivate_1.0 {
+#ifdef PLATFORM_FOO
+	_syscall1;
+	__sys_syscall1;
+#else
+#endif
+#ifdef PLATFORM_FOO
+#else
+	_syscall2;
+	__sys_syscall2;
+#endif
+};
diff --git a/sys/tools/syscalls/test/test_sysent.c b/sys/tools/syscalls/test/test_sysent.c
new file mode 100644
index 00000000000000..18d2b0fd97121e
--- /dev/null
+++ b/sys/tools/syscalls/test/test_sysent.c
@@ -0,0 +1,26 @@
+/*
+ * System call switch table.
+ *
+ * DO NOT EDIT-- this file is automatically @generated.
+ */
+
+#include <sys/param.h>
+#include <sys/sysent.h>
+#include <sys/sysproto.h>
+
+#define AS(name) (sizeof(struct name) / sizeof(syscallarg_t))
+
+/* The casts are bogus but will do for now. */
+struct sysent test_sysent[] = {
+	{ .sy_narg = 0, .sy_call = (sy_call_t *)nosys, .sy_auevent = AUE_NULL, .sy_flags = 0, .sy_thrcnt = SY_THR_ABSENT },	/* 0 = unimpl_syscall0 */
+#ifdef PLATFORM_FOO
+	{ .sy_narg = AS(syscall1_args), .sy_call = (sy_call_t *)sys_syscall1, .sy_auevent = AUE_NULL, .sy_flags = 0, .sy_thrcnt = SY_THR_STATIC },	/* 1 = syscall1 */
+#else
+	{ .sy_narg = 0, .sy_call = (sy_call_t *)nosys, .sy_auevent = AUE_NULL, .sy_flags = 0, .sy_thrcnt = SY_THR_ABSENT },	/* 1 = reserved for local use */
+#endif
+#ifdef PLATFORM_FOO
+	{ .sy_narg = 0, .sy_call = (sy_call_t *)nosys, .sy_auevent = AUE_NULL, .sy_flags = 0, .sy_thrcnt = SY_THR_ABSENT },	/* 2 = obsolete syscall2 */
+#else
+	{ .sy_narg = 0, .sy_call = (sy_call_t *)sys_syscall2, .sy_auevent = AUE_NULL, .sy_flags = 0, .sy_thrcnt = SY_THR_STATIC },	/* 2 = syscall2 */
+#endif
+};
diff --git a/sys/tools/syscalls/test/test_systrace_args.c b/sys/tools/syscalls/test/test_systrace_args.c
new file mode 100644
index 00000000000000..74d0aa0ee8623a
--- /dev/null
+++ b/sys/tools/syscalls/test/test_systrace_args.c
@@ -0,0 +1,91 @@
+/*
+ * System call argument to DTrace register array conversion.
+ *
+ * This file is part of the DTrace syscall provider.
+ *
+ * DO NOT EDIT-- this file is automatically @generated.
+ */
+
+static void
+systrace_args(int sysnum, void *params, uint64_t *uarg, int *n_args)
+{
+	int64_t *iarg = (int64_t *)uarg;
+	int a = 0;
+	switch (sysnum) {
+#ifdef PLATFORM_FOO
+	/* syscall1 */
+	case 1: {
+		struct syscall1_args *p = params;
+		iarg[a++] = p->arg1; /* int */
+		*n_args = 1;
+		break;
+	}
+#else
+#endif
+#ifdef PLATFORM_FOO
+#else
+	/* syscall2 */
+	case 2: {
+		*n_args = 0;
+		break;
+	}
+#endif
+	default:
+		*n_args = 0;
+		break;
+	};
+}
+static void
+systrace_entry_setargdesc(int sysnum, int ndx, char *desc, size_t descsz)
+{
+	const char *p = NULL;
+	switch (sysnum) {
+#ifdef PLATFORM_FOO
+	/* syscall1 */
+	case 1:
+		switch (ndx) {
+		case 0:
+			p = "int";
+			break;
+		default:
+			break;
+		};
+		break;
+#else
+#endif
+#ifdef PLATFORM_FOO
+#else
+	/* syscall2 */
+	case 2:
+		break;
+#endif
+	default:
+		break;
+	};
+	if (p != NULL)
+		strlcpy(desc, p, descsz);
+}
+static void
+systrace_return_setargdesc(int sysnum, int ndx, char *desc, size_t descsz)
+{
+	const char *p = NULL;
+	switch (sysnum) {
+#ifdef PLATFORM_FOO
+	/* syscall1 */
+	case 1:
+		if (ndx == 0 || ndx == 1)
+			p = "int";
+		break;
+#else
+#endif
+#ifdef PLATFORM_FOO
+#else
+	/* syscall2 */
+	case 2:
+#endif
+	default:
+		break;
+	};
+	if (p != NULL)
+		strlcpy(desc, p, descsz);
+}
diff --git a/sys/tools/syscalls/tools/util.lua b/sys/tools/syscalls/tools/util.lua
index c9ff98dda786ff..41247e34a10e9f 100644
--- a/sys/tools/syscalls/tools/util.lua
+++ b/sys/tools/syscalls/tools/util.lua
@@ -35,6 +35,11 @@ function util.split(s, re)
 	return t
 end
 
+-- Prints a warning to stderr
+function util.warn(msg)
+	assert(io.stderr:write("WARNING: " .. msg .. "\n"))
+end
+
 -- Aborts with a message and does a clean exit procedure.
 function util.abort(status, msg)
 	assert(io.stderr:write(msg .. "\n"))