lib/vdso: Provide generic VDSO implementation

In the last few years the kernel gained quite some architecture specific
vdso implementations which contain very similar code.

Introduce a generic VDSO implementation of gettimeofday() which will be
shareable between architectures once they are converted over.

The implementation is based on the current x86 VDSO code.

[ tglx: Massaged changelog and made the kernel doc tabular ]

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Shijith Thotton <sthotton@marvell.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>
Cc: linux-arch@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mips@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Mark Salyzyn <salyzyn@android.com>
Cc: Peter Collingbourne <pcc@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Huw Davies <huw@codeweavers.com>
Link: https://lkml.kernel.org/r/20190621095252.32307-3-vincenzo.frascino@arm.com

lib/vdso/Kconfig

@@ -0,0 +1,36 @@
+# SPDX-License-Identifier: GPL-2.0
+
+config HAVE_GENERIC_VDSO
+	bool
+
+if HAVE_GENERIC_VDSO
+
+config GENERIC_GETTIMEOFDAY
+	bool
+	help
+	  This is a generic implementation of gettimeofday vdso.
+	  Each architecture that enables this feature has to
+	  provide the fallback implementation.
+
+config GENERIC_VDSO_32
+	bool
+	depends on GENERIC_GETTIMEOFDAY && !64BIT
+	help
+	  This config option helps to avoid possible performance issues
+	  in 32 bit only architectures.
+
+config GENERIC_COMPAT_VDSO
+	bool
+	help
+	  This config option enables the compat VDSO layer.
+
+config CROSS_COMPILE_COMPAT_VDSO
+	string "32 bit Toolchain prefix for compat vDSO"
+	default ""
+	depends on GENERIC_COMPAT_VDSO
+	help
+	  Defines the cross-compiler prefix for compiling compat vDSO.
+	  If a 64 bit compiler (i.e. x86_64) can compile the VDSO for
+	  32 bit, it does not need to define this parameter.
+
+endif