1. 16 Jan, 2015 1 commit
    • Andy Lutomirski's avatar
      x86, vdso: Use asm volatile in __getcpu · d1d9e22b
      Andy Lutomirski authored
      commit 1ddf0b1b upstream.
      
      In Linux 3.18 and below, GCC hoists the lsl instructions in the
      pvclock code all the way to the beginning of __vdso_clock_gettime,
      slowing the non-paravirt case significantly.  For unknown reasons,
      presumably related to the removal of a branch, the performance issue
      is gone as of
      
      e76b027e x86,vdso: Use LSL unconditionally for vgetcpu
      
      but I don't trust GCC enough to expect the problem to stay fixed.
      
      There should be no correctness issue, because the __getcpu calls in
      __vdso_vlock_gettime were never necessary in the first place.
      
      Note to stable maintainers: In 3.18 and below, depending on
      configuration, gcc 4.9.2 generates code like this:
      
           9c3:       44 0f 03 e8             lsl    %ax,%r13d
           9c7:       45 89 eb                mov    %r13d,%r11d
           9ca:       0f 03 d8                lsl    %ax,%ebx
      
      This patch won't apply as is to any released kernel, but I'll send a
      trivial backported version if needed.
      
      [
       Backported by Andy Lutomirski.  Should apply to all affected
       versions.  This fixes a functionality bug as well as a performance
       bug: buggy kernels can infinite loop in __vdso_clock_gettime on
       affected compilers.  See, for exammple:
      
       https://bugzilla.redhat.com/show_bug.cgi?id=1178975
      ]
      
      Fixes: 51c19b4f x86: vdso: pvclock gettime support
      Cc: Marcelo Tosatti <mtosatti@redhat.com>
      Acked-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
      Signed-off-by: default avatarAndy Lutomirski <luto@amacapital.net>
      [ luis: backported to 3.16: used Andy's backport for stable kernels ]
      Signed-off-by: default avatarLuis Henriques <luis.henriques@canonical.com>
      d1d9e22b
  2. 15 Jan, 2015 39 commits