Add simd_config.h and HAVE_SIMD() selector

We need to select which SIMD variable to check based on the compilation
target: HAVE_KERNEL_xxx for the Linux kernel, HAVE_TOOLCHAIN_xxx for
other platforms.

This adds a HAVE_SIMD() macro returns the right result depending on the
definedness or value of the variable for this target.

The macro is in simd_config.h, which is forcibly included in every
compiler call (like zfs_config.h), to ensure that it can be used
directly without further includes.

Sponsored-by: TrueNAS
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@truenas.com>
Closes #18285

config/Rules.am

@@ -7,6 +7,7 @@
 PHONY =
 AM_CPPFLAGS = \
 	-include $(top_builddir)/zfs_config.h \
+	-include $(top_srcdir)/include/sys/simd_config.h \
 	-I$(top_builddir)/include \
 	-I$(top_srcdir)/include \
 	-I$(top_srcdir)/lib/libspl/include \

include/Makefile.am

@@ -82,6 +82,7 @@ COMMON_H = \
 	sys/sa.h \
 	sys/sa_impl.h \
 	sys/sha2.h \
+	sys/simd_config.h \
 	sys/skein.h \
 	sys/spa.h \
 	sys/spa_checkpoint.h \

include/sys/simd_config.h

@@ -0,0 +1,102 @@
+// SPDX-License-Identifier: CDDL-1.0
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or https://opensource.org/licenses/CDDL-1.0.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright (c) 2026, TrueNAS.
+ */
+
+#ifndef _SYS_SIMD_CONFIG_H
+#define	_SYS_SIMD_CONFIG_H
+
+/*
+ * Goal: a HAVE_SIMD(x) macro that expands to either 1 or 0 depending on
+ * the availabilty of that extension on the wanted toolchain.
+ *
+ * We need to allow the following:
+ * - #define HAVE_TOOLCHAIN_AVX 1   (zfs_config.h, detected by configure)
+ * - #undef HAVE_TOOLCHAIN_AVX      (zfs_config.h, probed but not detected)
+ * - -DHAVE_TOOLCHAIN_AVX           (Makefile.bsd)
+ *
+ * For completeness, we also allow disabling by defined HAVE_TOOLCHAIN_AVX=0.
+ *
+ * The "obvious" implementation of this would be a macro that includes
+ * defined(...) in its expansion, but unfortunately that is not portable, and
+ * can produce compiler warnings (see -Wexpansion-to-defined). So we need to
+ * build our own version.
+ */
+
+/*
+ * 1. Expand incoming token to its defined value, if anything:
+ *    eg HAVE_SIMD(AVX)
+ *	-> _zfs_deftrue(HAVE_TOOLCHAIN_AVX)
+ *	or -> __zfs_deftrue()			    (HAVE_TOOLCHAIN_AVX defined)
+ *	or -> __zfs_deftrue(1)			    (HAVE_TOOLCHAIN_AVX = 1)
+ *	   -> __zfs_deftrue(0)			    (HAVE_TOOLCHAIN_AVX = 0)
+ *	or -> __zfs_deftrue(HAVE_TOOLCHAIN_AVX)	    (HAVE_TOOLCHAIN_AVX undef)
+ */
+#define	_zfs_deftrue(x)		__zfs_deftrue(x)
+
+/*
+ * 2. Replace know values with a token that we control:
+ *    __zfs_deftrue()
+ *	-> ___zfs_deftrue(___zfs_deftrue_arg_)
+ *    __zfs_deftrue(1)
+ *	-> ___zfs_deftrue(___zfs_deftrue_arg_1)
+ *    __zfs_deftrue(0)
+ *	-> ___zfs_deftrue(___zfs_deftrue_arg_0)
+ *    __zfs_deftrue(HAVE_TOOLCHAIN_AVX)
+ *	-> ___zfs_deftrue(___zfs_deftrue_arg_HAVE_TOOLCHAIN_AVX)
+ */
+#define	__zfs_deftrue(v)	___zfs_deftrue(___zfs_deftrue_arg_##v)
+
+/*
+ * 3. Expand the incoming token into positional parameters for the next call:
+ *    ___zfs_deftrue(___zfs_deftrue_arg_0)
+ *	-> ____zfs_deftrue(0, 0, 0)
+ *    ___zfs_deftrue(___zfs_deftrue_arg_1)
+ *	-> ____zfs_deftrue(0, 1, 0)
+ *    ___zfs_deftrue(___zfs_deftrue_arg_HAVE_TOOLCHAIN_AVX)
+ *	-> ____zfs_deftrue(___zfs_deftrue_arg_HAVE_TOOLCHAIN_AVX, 0)
+ */
+#define	___zfs_deftrue_arg_		0, 1
+#define	___zfs_deftrue_arg_1		0, 1
+#define	___zfs_deftrue_arg_0		0, 0
+#define	___zfs_deftrue(t, ...)		____zfs_deftrue(t, 0)
+
+/*
+ * 4. Emit the second argument, either the original value or the default 0.
+ *    ____zfs_deftrue(0, 0, 0)					-> 0
+ *    ____zfs_deftrue(0, 1, 0)					-> 1
+ *    ____zfs_deftrue(___zfs_deftrue_arg_HAVE_TOOLCHAIN_AVX, 0)	-> 0
+ */
+#define	____zfs_deftrue(_n, v, ...)	v
+
+/*
+ * The Linux kernel requires a specific toolchain. Everything else uses the
+ * regular compiler toolchain.
+ */
+#if defined(_KERNEL) && defined(__linux__)
+#define	HAVE_SIMD(ext)	_zfs_deftrue(HAVE_KERNEL_##ext)
+#else
+#define	HAVE_SIMD(ext)	_zfs_deftrue(HAVE_TOOLCHAIN_##ext)
+#endif
+
+#endif

module/Kbuild.in

@@ -11,6 +11,7 @@ zfs_include = @abs_top_srcdir@/include
 icp_include = @abs_srcdir@/icp/include
 zstd_include = @abs_srcdir@/zstd/include
 ZFS_MODULE_CFLAGS += -include @abs_top_builddir@/zfs_config.h
+ZFS_MODULE_CFLAGS += -include $(zfs_include)/sys/simd_config.h
 ZFS_MODULE_CFLAGS += -I@abs_top_builddir@/include
 src = @abs_srcdir@
 obj = @abs_builddir@
@@ -19,6 +20,7 @@ zfs_include = $(srctree)/include/zfs
 icp_include = $(src)/icp/include
 zstd_include = $(src)/zstd/include
 ZFS_MODULE_CFLAGS += -include $(zfs_include)/zfs_config.h
+ZFS_MODULE_CFLAGS += -include $(zfs_include)/sys/simd_config.h
 endif
 
 ZFS_MODULE_CFLAGS += -I$(zfs_include)/os/linux/kernel

module/Makefile.bsd

@@ -38,6 +38,7 @@ CFLAGS+= -I${INCDIR}/os/freebsd
 CFLAGS+= -I${INCDIR}/os/freebsd/spl
 CFLAGS+= -I${INCDIR}/os/freebsd/zfs
 CFLAGS+= -I${SRCDIR}/zstd/include
+CFLAGS+= -include ${INCDIR}/sys/simd_config.h
 CFLAGS+= -include ${INCDIR}/os/freebsd/spl/sys/ccompile.h
 CFLAGS+= -I${.CURDIR}
 
@@ -46,8 +47,10 @@ CFLAGS+= -D__KERNEL__ -DFREEBSD_NAMECACHE -DBUILDING_ZFS -D__BSD_VISIBLE=1 \
 	-D_SYS_VMEM_H_ -DKDTRACE_HOOKS -DCOMPAT_FREEBSD11
 
 .if ${MACHINE_ARCH} == "amd64"
-CFLAGS+= -D__x86_64 -DHAVE_SSE2 -DHAVE_SSSE3 -DHAVE_SSE4_1 \
+CFLAGS+= -D__x86_64 -DHAVE_TOOLCHAIN_SSE2 -DHAVE_TOOLCHAIN_SSSE3 \
-	-DHAVE_AVX -DHAVE_AVX2 -DHAVE_AVX512F -DHAVE_AVX512VL -DHAVE_AVX512BW
+	-DHAVE_TOOLCHAIN_SSE4_1 -DHAVE_TOOLCHAIN_AVX -DHAVE_TOOLCHAIN_AVX2 \
+	-DHAVE_TOOLCHAIN_AVX512F -DHAVE_TOOLCHAIN_AVX512VL \
+	-DHAVE_TOOLCHAIN_AVX512BW
 .endif
 
 .if defined(WITH_DEBUG) && ${WITH_DEBUG} == "true"