• Andi Kleen's avatar
    x86_64: Add vDSO for x86-64 with gettimeofday/clock_gettime/getcpu · 2aae950b
    Andi Kleen authored
    This implements new vDSO for x86-64.  The concept is similar
    to the existing vDSOs on i386 and PPC.  x86-64 has had static
    vsyscalls before,  but these are not flexible enough anymore.
    
    A vDSO is a ELF shared library supplied by the kernel that is mapped into
    user address space.  The vDSO mapping is randomized for each process
    for security reasons.
    
    Doing this was needed for clock_gettime, because clock_gettime
    always needs a syscall fallback and having one at a fixed
    address would have made buffer overflow exploits too easy to write.
    
    The vdso can be disabled with vdso=0
    
    It currently includes a new gettimeofday implemention and optimized
    clock_gettime(). The gettimeofday implementation is slightly faster
    than the one in the old vsyscall.  clock_gettime is significantly faster
    than the syscall for CLOCK_MONOTONIC and CLOCK_REALTIME.
    
    The new calls are generally faster than the old vsyscall.
    
    Advantages over the old x86-64 vsyscalls:
    - Extensible
    - Randomized
    - Cleaner
    - Easier to virtualize (the old static address range previously causes
    overhead e.g. for Xen because it has to create special page tables for it)
    
    Weak points:
    - glibc support still to be written
    
    The VM interface is partly based on Ingo Molnar's i386 version.
    
    Includes compile fix from Joachim Deguara
    Signed-off-by: default avatarAndi Kleen <ak@suse.de>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    2aae950b
vclock_gettime.c 3.1 KB