1. 28 Jul, 2010 1 commit
    • Paul Mackerras's avatar
      powerpc: Rework VDSO gettimeofday to prevent time going backwards · 0e469db8
      Paul Mackerras authored
      Currently it is possible for userspace to see the result of
      gettimeofday() going backwards by 1 microsecond, assuming that
      userspace is using the gettimeofday() in the VDSO.  The VDSO
      gettimeofday() algorithm computes the time in "xsecs", which are
      units of 2^-20 seconds, or approximately 0.954 microseconds,
      using the algorithm
      
      	now = (timebase - tb_orig_stamp) * tb_to_xs + stamp_xsec
      
      and then converts the time in xsecs to seconds and microseconds.
      
      The kernel updates the tb_orig_stamp and stamp_xsec values every
      tick in update_vsyscall().  If the length of the tick is not an
      integer number of xsecs, then some precision is lost in converting
      the current time to xsecs.  For example, with CONFIG_HZ=1000, the
      tick is 1ms long, which is 1048.576 xsecs.  That means that
      stamp_xsec will advance by either 1048 or 1049 on each tick.
      With the right conditions, it is possible for userspace to get
      (timebase - tb_orig_stamp) * tb_to_xs being 1049 if the kernel is
      slightly late in updating the vdso_datapage, and then for stamp_xsec
      to advance by 1048 when the kernel does update it, and for userspace
      to then see (timebase - tb_orig_stamp) * tb_to_xs being zero due to
      integer truncation.  The result is that time appears to go backwards
      by 1 microsecond.
      
      To fix this we change the VDSO gettimeofday to use a new field in the
      VDSO datapage which stores the nanoseconds part of the time as a
      fractional number of seconds in a 0.32 binary fraction format.
      (Or put another way, as a 32-bit number in units of 0.23283 ns.)
      This is convenient because we can use the mulhwu instruction to
      convert it to either microseconds or nanoseconds.
      
      Since it turns out that computing the time of day using this new field
      is simpler than either using stamp_xsec (as gettimeofday does) or
      stamp_xtime.tv_nsec (as clock_gettime does), this converts both
      gettimeofday and clock_gettime to use the new field.  The existing
      __do_get_tspec function is converted to use the new field and take
      a parameter in r7 that indicates the desired resolution, 1,000,000
      for microseconds or 1,000,000,000 for nanoseconds.  The __do_get_xsec
      function is then unused and is deleted.
      
      The new algorithm is
      
      	now = ((timebase - tb_orig_stamp) << 12) * tb_to_xs
      		+ (stamp_xtime_seconds << 32) + stamp_sec_fraction
      
      with 'now' in units of 2^-32 seconds.  That is then converted to
      seconds and either microseconds or nanoseconds with
      
      	seconds = now >> 32
      	partseconds = ((now & 0xffffffff) * resolution) >> 32
      
      The 32-bit VDSO code also makes a further simplification: it ignores
      the bottom 32 bits of the tb_to_xs value, which is a 0.64 format binary
      fraction.  Doing so gets rid of 4 multiply instructions.  Assuming
      a timebase frequency of 1GHz or less and an update interval of no
      more than 10ms, the upper 32 bits of tb_to_xs will be at least
      4503599, so the error from ignoring the low 32 bits will be at most
      2.2ns, which is more than an order of magnitude less than the time
      taken to do gettimeofday or clock_gettime on our fastest processors,
      so there is no possibility of seeing inconsistent values due to this.
      
      This also moves update_gtod() down next to its only caller, and makes
      update_vsyscall use the time passed in via the wall_time argument rather
      than accessing xtime directly.  At present, wall_time always points to
      xtime, but that could change in future.
      Signed-off-by: default avatarPaul Mackerras <paulus@samba.org>
      Signed-off-by: default avatarBenjamin Herrenschmidt <benh@kernel.crashing.org>
      0e469db8
  2. 26 Jul, 2010 39 commits