[RFC PATCH] powerpc: Add the ppc_capabilities ELF note

[Claudio Carvalho <cclaudio@linux.ibm.com>]

Add the ppc_capabilities ELF note to the powerpc kernel binary. It is a
bitmap that can be used to advertise kernel capabilities to userland.

This patch also defines PPCCAP_ULTRAVISOR_BIT as being the bit zero.

Suggested-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Claudio Carvalho <cclaudio@linux.ibm.com>
---
 arch/powerpc/kernel/Makefile |  2 +-
 arch/powerpc/kernel/note.S   | 36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)
 create mode 100644 arch/powerpc/kernel/note.S

diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 0ea6c4aa3a20..4ec36fe4325b 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -49,7 +49,7 @@ obj-y				:= cputable.o ptrace.o syscalls.o \
 				   signal.o sysfs.o cacheinfo.o time.o \
 				   prom.o traps.o setup-common.o \
 				   udbg.o misc.o io.o misc_$(BITS).o \
-				   of_platform.o prom_parse.o
+				   of_platform.o prom_parse.o note.o
 obj-$(CONFIG_PPC64)		+= setup_64.o sys_ppc32.o \
 				   signal_64.o ptrace32.o \
 				   paca.o nvram_64.o firmware.o
diff --git a/arch/powerpc/kernel/note.S b/arch/powerpc/kernel/note.S
new file mode 100644
index 000000000000..721bf8ce9eb7
--- /dev/null
+++ b/arch/powerpc/kernel/note.S
@@ -0,0 +1,36 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * PowerPC ELF notes.
+ *
+ * Copyright 2019, IBM Corporation
+ */
+#include <linux/elfnote.h>
+
+/*
+ * Ultravisor-capable bit (PowerNV only).
+ *
+ * Indicate that the powerpc kernel binary knows how to run in an
+ * ultravisor-enabled system.
+ *
+ * In an ultravisor-enabled system, some machine resources are now controlled
+ * by the ultravisor. If the kernel is not ultravisor-capable, but it ends up
+ * being run on a machine with ultravisor, the kernel will probably crash
+ * trying to access ultravisor resources. For instance, it may crash in early
+ * boot trying to set the partition table entry 0.
+ *
+ * In an ultravisor-enabled system, petitboot can warn the user or prevent the
+ * kernel from being run if the ppc_capabilities doesn't exist or the
+ * Ultravisor-capable bit is not set.
+ */
+#if defined(CONFIG_PPC_POWERNV)
+#define PPCCAP_ULTRAVISOR_BIT		(1 << 0)
+#else
+#define PPCCAP_ULTRAVISOR_BIT		0
+#endif
+
+/*
+ * Add the ppc_capabilities ELF note to the powerpc kernel binary. It is a
+ * bitmap that can be used to advertise kernel capabilities to userland.
+ */
+ELFNOTE(ppc_capabilities, 3,
+	.long PPCCAP_ULTRAVISOR_BIT)
-- 
2.20.1