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>

1 files changed, 80 insertions, 0 deletions

diff --git a/arch/mips/vdso/vdso.h b/arch/mips/vdso/vdso.h
new file mode 100644
index 000000000000..0bb6b1adc385
--- /dev/null
+++ b/arch/mips/vdso/vdso.h
@@ -0,0 +1,80 @@
+/*
+ * Copyright (C) 2015 Imagination Technologies
+ * Author: Alex Smith <alex.smith@imgtec.com>
+ *
+ * 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.
+ */
+
+#include <asm/sgidefs.h>
+
+#if _MIPS_SIM != _MIPS_SIM_ABI64 && defined(CONFIG_64BIT)
+
+/* Building 32-bit VDSO for the 64-bit kernel. Fake a 32-bit Kconfig. */
+#undef CONFIG_64BIT
+#define CONFIG_32BIT 1
+#ifndef __ASSEMBLY__
+#include <asm-generic/atomic64.h>
+#endif
+#endif
+
+#ifndef __ASSEMBLY__
+
+#include <asm/asm.h>
+#include <asm/page.h>
+#include <asm/vdso.h>
+
+static inline unsigned long get_vdso_base(void)
+{
+	unsigned long addr;
+
+	/*
+	 * We can't use cpu_has_mips_r6 since it needs the cpu_data[]
+	 * kernel symbol.
+	 */
+#ifdef CONFIG_CPU_MIPSR6
+	/*
+	 * lapc <symbol> is an alias to addiupc reg, <symbol> - .
+	 *
+	 * We can't use addiupc because there is no label-label
+	 * support for the addiupc reloc
+	 */
+	__asm__("lapc	%0, _start			\n"
+		: "=r" (addr) : :);
+#else
+	/*
+	 * Get the base load address of the VDSO. We have to avoid generating
+	 * relocations and references to the GOT because ld.so does not peform
+	 * relocations on the VDSO. We use the current offset from the VDSO base
+	 * and perform a PC-relative branch which gives the absolute address in
+	 * ra, and take the difference. The assembler chokes on
+	 * "li %0, _start - .", so embed the offset as a word and branch over
+	 * it.
+	 *
+	 */
+
+	__asm__(
+	"	.set push				\n"
+	"	.set noreorder				\n"
+	"	bal	1f				\n"
+	"	 nop					\n"
+	"	.word	_start - .			\n"
+	"1:	lw	%0, 0($31)			\n"
+	"	" STR(PTR_ADDU) " %0, $31, %0		\n"
+	"	.set pop				\n"
+	: "=r" (addr)
+	:
+	: "$31");
+#endif /* CONFIG_CPU_MIPSR6 */
+
+	return addr;
+}
+
+static inline const union mips_vdso_data *get_vdso_data(void)
+{
+	return (const union mips_vdso_data *)(get_vdso_base() - PAGE_SIZE);
+}
+
+#endif /* __ASSEMBLY__ */