[PATCH 0/4] Add 32 bit VDSO time function support

stefani
2014-02-01T15:32:00Z

From: Stefani Seibold <stefani@seibold.net>

This patch add the functions vdso_gettimeofday(), vdso_clock_gettime()
and vdso_time() to the 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

This kind of helper must be integrated into glibc, for x86 64 bit and
PowerPC it is already there.

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 address layout of the VDSO has changed, because there is no fixed
address space available on a x86 32 bit kernel, despite the name. Because
someone decided to add an offset to the __FIXADDR_TOP for virtualization.

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

This was the reason not depend on this kind of address and change the layout
of the VDSO. The VDSO for a 32 bit application has now three pages:

+----------------------------------------+
+ VDSO page (includes code) ro+x +
+----------------------------------------+
+ VVAR page (export kernel variables) ro +
+----------------------------------------+
+ HPET page (mapped registers) ro
+----------------------------------------+

The VDSO page for a 32 bit resided now on 0xffffc000, followed by the VVAR and
HPET page.

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 kernel 3.14 (e7651b819e90da924991d727d3c007200a18670d)

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
30.01.2013 - revamp the code
- code clean up
- VDSO layout changed
- no fixed addresses
- port to 3.14
01.02.2013 - code cleanup