[PATCH v23 00/13] x86: Add x86 32 bit VDSO time function support

[Stefani Seibold]

This patch add the functions vdso_gettimeofday(), vdso_clock_gettime()
and vdso_time() to the x86 32 bit VDSO.

The reason to do this was to get a fast reliable time stamp. Many developers
uses TSC to get a fast time stamp, without knowing the pitfalls. VDSO
time functions a fast and a reliable way, because the kernel knows the
best time source and the P- and C-state of the CPU.

The helper library to use the VDSO functions can be download at
http://http://seibold.net/vdso.c
The libary is very small, only 228 lines of code. Compile it with
gcc -Wall -O3 -fpic vdso.c -lrt -shared -o libvdso.so
and use it with LD_PRELOAD=<path>/libvdso.so

There is also a patch http://seibold.net/glibc.patch for glibc 2.19 which also
works for glibc 2.18.  This patch must be integrated into glibc.

Some linux 32 bit kernel benchmark results (all measurements are in nano
seconds):

Intel(R) Celeron(TM) CPU 400MHz

Average time kernel call:
 gettimeofday(): 1039
 clock_gettime(): 1578
 time(): 526
Average time VDSO call:
 gettimeofday(): 378
 clock_gettime(): 303
 time(): 60

Celeron(R) Dual-Core CPU T3100 1.90GHz

Average time kernel call:
 gettimeofday(): 209
 clock_gettime(): 406
 time(): 135
Average time VDSO call:
 gettimeofday(): 51
 clock_gettime(): 43
 time(): 10

So you can see a performance increase between 4 and 13, depending on the
CPU and the function.

The IA32 Emulation uses the whole 4 GB address space, so there is no
fixed address available.

The VDSO for a 32 bit application has now three pages:

^ Higher Address
|
+----------------------------------------+
+ VDSO page (includes code) ro+x         +
+----------------------------------------+
+ VVAR page (export kernel variables) ro +
+----------------------------------------+
+ HPET page (mapped registers) ro 
+----------------------------------------+
|
^ Lower Address

The VDSO page for a 32 bit resided still on 0xffffe000, the the VVAR and
HPET page are mapped before.

In the non compat mode the VMA of the VDSO is now 3 pages for a 32 bit kernel.
So this decrease the available logical address room by 2 pages.

The patch is against tip x86/vdso (1f2cbcf648962cdcf511d234cb39745baa9f5d07)

Changelog:
25.11.2012 - first release and proof of concept for linux 3.4
11.12.2012 - Port to linux 3.7 and code cleanup
12.12.2012 - fixes suggested by Andy Lutomirski
           - fixes suggested by John Stultz
           - use call VDSO32_vsyscall instead of int 80
           - code cleanup
17.12.2012 - support for IA32_EMULATION, this includes
             - code cleanup
             - include cleanup to fix compile warnings and errors
             - move out seqcount from seqlock, enable use in VDSO
             - map FIXMAP and HPET into the 32 bit address space
18.12.2012 - split into separate patches
30.01.2014 - revamp the code
             - code clean up
             - VDSO layout changed
             - no fixed addresses
             - port to 3.14
01.02.2014 - code cleanup
02.02.2014 - code cleanup
             - split into more patches
             - use HPET_COUNTER instead of hard coded value
             - fix changelog to the right year ;-)
02.02.2014 - reverse the mapping, this make the new VDSO 32 bit support
             full compatible.
03.02.2014 - code cleanup
             - fix comment
             - fix ABI break in vdso32.lds.S
04.02.2014 - revamp IA32 emulation support
             - introduce VVAR macro
             - rearranged vsyscall_gtod_data struture for IA32 emulation support
             - code cleanup
05.02.2014 - revamp IA32 emulation support
             - replace seqcount_t by an unsigned, to make the vsyscall_gtod_data
               structure independed of kernel config and functions.
08.02.2014 - revamp IA32 emulation support
             - replace all internal structures by fix size elements
10.02.2014 - code cleanup
             - add commets
             - revamp inline assembly
12.02.2014 - add conditional fixmap of vvar and hpet pages for 32 bit kernel
14.02.2014 - fix CONFIG_PARAVIRT_CLOCK, which is not supported in 32 bit VDSO
15.02.2014 - fix tsc
             code cleanup
             tested make ARCH=i386 allyesconfig and make allyesconfig
16.02.2014 - code cleanup
             - fix all C=1 warnings, also some one not introduced by this patch
             - hack to fix C=1 32 bit VDSO spinlock for a 64 bit kernel
             - fix VDSO Makefile for newer gcc
             tested for gcc 4.3.4 and 4.8.1
             tested ARCH=i386 allyesconfig, defconfig and allmodconfig
             tested X86_64 allyesconfig, defconfig and allmodconfig
17.02.2014 - In case of a 32 bit VDSO for a 64 bit kernel fake a 32 bit kernel
             configuration.
19.02.2014 - Add missing #undef and #define to fake proper 32 bit kernel config
             Add a missing #ifdef CONFIG_HPET_TIMER
             tested again ARCH=i386 allyesconfig, defconfig and allmodconfig
             tested again ARCH=X86_64 allyesconfig, defconfig and allmodconfig
02.03.2014 - Add fixes suggested by Andy Lutomirski
             - Patch alternatives in the 32 bit VDSO
	     - Use the default ABI for the 32 bit VDSO
	     - Inline the CLOCK MONOTONIC VDSO code
	     - Zero pad the VVAR page
	     - fix "patch alternatives" compile for 32 bit kernel
03.03.2014 - Add glibc.patch http://seibold.net/glibc.patch
             Add reviewed-by tags
17.03.2014 - Rebase on tip x86/vdso 1f2cbcf648962cdcf511d234cb39745baa9f5d07
             remove fixmap and compat mode dependencies
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