MIPS: Initial implementation of a VDSO

Add an initial implementation of a proper (i.e. an ELF shared library)
VDSO. With this commit it does not export any symbols, it only replaces
the current signal return trampoline page. A later commit will add user
implementations of gettimeofday()/clock_gettime().

To support both new toolchains and old ones which don't generate ABI
flags section, we define its content manually and then use a tool
(genvdso) to patch up the section to have the correct name and type.
genvdso also extracts symbol offsets ({,rt_}sigreturn) needed by the
kernel, and generates a C file containing a "struct mips_vdso_image"
containing both the VDSO data and these offsets. This C file is
compiled into the kernel.

On 64-bit kernels we require a different VDSO for each supported ABI,
so we may build up to 3 different VDSOs. The VDSO to use is selected by
the mips_abi structure.

A kernel/user shared data page is created and mapped below the VDSO
image. This is currently empty, but will be used by the user time
function implementations which are added later.

[markos.chandras@imgtec.com:
- Add more comments
- Move abi detection in genvdso.h since it's the get_symbol function
that needs it.
- Add an R6 specific way to calculate the base address of VDSO in order
to avoid the branch instruction which affects performance.
- Do not patch .gnu.attributes since it's not needed for dynamic linking.
- Simplify Makefile a little bit.
- checkpatch fixes
- Restrict VDSO support for binutils < 2.25 for pre-R6
- Include atomic64.h for O32 variant on MIPS64]

Signed-off-by: Alex Smith <alex.smith@imgtec.com>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: Matthew Fortune <matthew.fortune@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11337/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/vdso.h

@@ -1,29 +1,70 @@
 /*
- * This file is subject to the terms and conditions of the GNU General Public
- * License.  See the file "COPYING" in the main directory of this archive
- * for more details.
+ * Copyright (C) 2015 Imagination Technologies
+ * Author: Alex Smith <alex.smith@imgtec.com>
  *
- * Copyright (C) 2009 Cavium Networks
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation;  either version 2 of the  License, or (at your
+ * option) any later version.
  */
 
 #ifndef __ASM_VDSO_H
 #define __ASM_VDSO_H
 
-#include <linux/types.h>
+#include <linux/mm_types.h>
 
+/**
+ * struct mips_vdso_image - Details of a VDSO image.
+ * @data: Pointer to VDSO image data (page-aligned).
+ * @size: Size of the VDSO image data (page-aligned).
+ * @off_sigreturn: Offset of the sigreturn() trampoline.
+ * @off_rt_sigreturn: Offset of the rt_sigreturn() trampoline.
+ * @mapping: Special mapping structure.
+ *
+ * This structure contains details of a VDSO image, including the image data
+ * and offsets of certain symbols required by the kernel. It is generated as
+ * part of the VDSO build process, aside from the mapping page array, which is
+ * populated at runtime.
+ */
+struct mips_vdso_image {
+	void *data;
+	unsigned long size;
 
-#ifdef CONFIG_32BIT
-struct mips_vdso {
-	u32 signal_trampoline[2];
-	u32 rt_signal_trampoline[2];
+	unsigned long off_sigreturn;
+	unsigned long off_rt_sigreturn;
+
+	struct vm_special_mapping mapping;
 };
-#else  /* !CONFIG_32BIT */
-struct mips_vdso {
-	u32 o32_signal_trampoline[2];
-	u32 o32_rt_signal_trampoline[2];
-	u32 rt_signal_trampoline[2];
-	u32 n32_rt_signal_trampoline[2];
+
+/*
+ * The following structures are auto-generated as part of the build for each
+ * ABI by genvdso, see arch/mips/vdso/Makefile.
+ */
+
+extern struct mips_vdso_image vdso_image;
+
+#ifdef CONFIG_MIPS32_O32
+extern struct mips_vdso_image vdso_image_o32;
+#endif
+
+#ifdef CONFIG_MIPS32_N32
+extern struct mips_vdso_image vdso_image_n32;
+#endif
+
+/**
+ * union mips_vdso_data - Data provided by the kernel for the VDSO.
+ *
+ * This structure contains data needed by functions within the VDSO. It is
+ * populated by the kernel and mapped read-only into user memory.
+ *
+ * Note: Care should be taken when modifying as the layout must remain the same
+ * for both 64- and 32-bit (for 32-bit userland on 64-bit kernel).
+ */
+union mips_vdso_data {
+	struct {
+	};
+
+	u8 page[PAGE_SIZE];
 };
-#endif /* CONFIG_32BIT */
 
 #endif /* __ASM_VDSO_H */