powerpc only is macppc these days, but not all macppc have altivec.
The following diff exports the bare minimum for altivec to be checked
by userland through elf_aux_info.  This could be enough for pixman and
mesa on OpenBSD to use the same code path as FreeBSD.  I propose this
as a first step so that someone with more knowledge of that
architecture can build upon.

Compile-tested only using clang-16 -target powerpc-unknown-openbsd.

Feedback welcome.  ok?


Index: arch/macppc/macppc/cpu.c
===================================================================
RCS file: /home/cvs/src/sys/arch/macppc/macppc/cpu.c,v
diff -u -p -r1.87 cpu.c
--- arch/macppc/macppc/cpu.c	24 Oct 2023 13:20:10 -0000	1.87
+++ arch/macppc/macppc/cpu.c	15 Jul 2024 11:59:39 -0000
@@ -48,6 +48,7 @@
 #include <powerpc/bat.h>
 #include <machine/cpu.h>
 #include <machine/trap.h>
+#include <machine/elf.h>
 #include <powerpc/hid.h>
 
 /* SCOM addresses (24-bit) */
@@ -201,6 +202,8 @@ cpuattach(struct device *parent, struct 
 	ci->ci_intrdepth = -1;
 	ci->ci_dev = dev;
 
+	hwcap = PPC_FEATURE_32 | PPC_FEATURE_HAS_FPU | PPC_FEATURE_HAS_MMU;
+
 	pvr = ppc_mfpvr();
 	cpu = pvr >> 16;
 	switch (cpu) {
@@ -278,6 +281,9 @@ cpuattach(struct device *parent, struct 
 #ifndef ALTIVEC			/* altivec support absent from kernel */
 	ppc_altivec = 0;
 #endif
+	if (ppc_altivec)
+		hwcap |= PPC_FEATURE_HAS_ALTIVEC;
+
 	snprintf(cpu_model + strlen(cpu_model),
 	    sizeof(cpu_model) - strlen(cpu_model),
 	    " (Revision 0x%x)", pvr & 0xffff);
Index: arch/powerpc/include/elf.h
===================================================================
RCS file: /home/cvs/src/sys/arch/powerpc/include/elf.h,v
diff -u -p -r1.1 elf.h
--- arch/powerpc/include/elf.h	14 Jul 2024 09:48:48 -0000	1.1
+++ arch/powerpc/include/elf.h	14 Jul 2024 15:28:50 -0000
@@ -1,14 +1,93 @@
 /*	$OpenBSD: elf.h,v 1.1 2024/07/14 09:48:48 jca Exp $	*/
 
-/*
- * This file is in the public domain.
+/*-
+ * SPDX-License-Identifier: BSD-4-Clause
+ *
+ * Copyright (C) 1995-1997 Wolfgang Solfrank.
+ * Copyright (C) 1995-1997 TooLs GmbH.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. All advertising materials mentioning features or use of this software
+ *    must display the following acknowledgement:
+ *	This product includes software developed by TooLs GmbH.
+ * 4. The name of TooLs GmbH may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY TOOLS GMBH ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL TOOLS GMBH BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
+ * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
+ * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ *	$NetBSD: cpu.h,v 1.11 2000/05/26 21:19:53 thorpej Exp $
  */
 
 #ifndef	_MACHINE_ELF_H_
 #define	_MACHINE_ELF_H_
 
 /*
- * TODO FreeBSD puts PPC_FEATURE* in cpu.h
+ * CPU Feature Attributes
+ *
+ * These are defined in the PowerPC ELF ABI for the AT_HWCAP vector,
+ * and are exported to userland via the elf_aux_info(3) function.
  */
 
-#endif /* !_MACHINE_ELF_H_ */
+#define __HAVE_HWCAP
+#define __HAVE_HWCAP2
+extern unsigned long hwcap;
+extern unsigned long hwcap2;
+
+#define	PPC_FEATURE_32		0x80000000	/* Always true */
+#define	PPC_FEATURE_64		0x40000000	/* Defined on a 64-bit CPU */
+#define	PPC_FEATURE_601_INSTR	0x20000000	/* Defined on a 64-bit CPU */
+#define	PPC_FEATURE_HAS_ALTIVEC	0x10000000	
+#define	PPC_FEATURE_HAS_FPU	0x08000000
+#define	PPC_FEATURE_HAS_MMU	0x04000000
+#define	PPC_FEATURE_UNIFIED_CACHE 0x01000000
+#define	PPC_FEATURE_HAS_SPE	0x00800000
+#define	PPC_FEATURE_HAS_EFP_SINGLE	0x00400000
+#define	PPC_FEATURE_HAS_EFP_DOUBLE	0x00200000
+#define	PPC_FEATURE_NO_TB	0x00100000
+#define	PPC_FEATURE_POWER4	0x00080000
+#define	PPC_FEATURE_POWER5	0x00040000
+#define	PPC_FEATURE_POWER5_PLUS	0x00020000
+#define	PPC_FEATURE_CELL	0x00010000
+#define	PPC_FEATURE_BOOKE	0x00008000
+#define	PPC_FEATURE_SMT		0x00004000
+#define	PPC_FEATURE_ICACHE_SNOOP	0x00002000
+#define	PPC_FEATURE_ARCH_2_05	0x00001000
+#define	PPC_FEATURE_HAS_DFP	0x00000400
+#define	PPC_FEATURE_POWER6_EXT	0x00000200
+#define	PPC_FEATURE_ARCH_2_06	0x00000100
+#define	PPC_FEATURE_HAS_VSX	0x00000080
+#define	PPC_FEATURE_TRUE_LE	0x00000002
+#define	PPC_FEATURE_PPC_LE	0x00000001
+
+#define	PPC_FEATURE2_ARCH_2_07	0x80000000
+#define	PPC_FEATURE2_HTM	0x40000000
+#define	PPC_FEATURE2_DSCR	0x20000000
+#define	PPC_FEATURE2_EBB	0x10000000
+#define	PPC_FEATURE2_ISEL	0x08000000
+#define	PPC_FEATURE2_TAR	0x04000000
+#define	PPC_FEATURE2_HAS_VEC_CRYPTO	0x02000000
+#define	PPC_FEATURE2_HTM_NOSC	0x01000000
+#define	PPC_FEATURE2_ARCH_3_00	0x00800000
+#define	PPC_FEATURE2_HAS_IEEE128	0x00400000
+#define	PPC_FEATURE2_DARN	0x00200000
+#define	PPC_FEATURE2_SCV	0x00100000
+#define	PPC_FEATURE2_HTM_NOSUSPEND	0x00080000
+
+#endif	/* _MACHINE_ELF_H_ */

-- 
jca