• Linus Torvalds's avatar
    Revert "x86-64: Disable local APIC timer use on AMD systems with C1E" · f7f847b0
    Linus Torvalds authored
    This reverts commit e66485d7, since
    Rafael Wysocki noticed that the change only works for his in -mm, not in
    mainline (and that both "noapictimer" _and_ "apicmaintimer" are broken
    on his hardware, but that's apparently not a regression, just a symptom
    of the same issue that causes the automatic apic timer disable to not
    work).
    
    It turns out that it really doesn't work correctly on x86-64, since
    x86-64 doesn't use the generic clock events for timers yet.
    
    Thanks to Rafal for testing, and here's the ugly details on x86-64 as
    per Thomas:
    
      "I just looked into the code and the logic vs.  noapictimer on SMP is
       completely broken.
    
       On i386 the noapictimer option not only disables the local APIC
       timer, it also registers the CPUs for broadcasting via IPI on SMP
       systems.
    
       The x86-64 code uses the broadcast only when the local apic timer is
       active, i.e.  "noapictimer" is not on the command line.  This defeats
       the whole purpose of "noapictimer".  It should be there to make boxen
       work, where the local APIC timer actually has a hardware problem,
       e.g.  the nx6325.
    
       The current implementation of x86_64 only fixes the ACPI c-states
       related problem where the APIC timer stops in C3(2), nothing else.
    
       On nx6325 and other AMD X2 equipped systems which have the C1E
       enabled we run into the following:
    
       PIT keeps jiffies (and the system) running, but the local APIC timer
       interrupts can get out of sync due to this C1E effect.
    
       I don't think this is a critical problem, but it is wrong
       nevertheless.
    
       I think it's safe to revert the C1E patch and postpone the fix to the
       clock events conversion."
    
    On further reflection, Thomas noted:
    
       "It's even worse than I thought on the first check:
    
        "noapictimer" on the command line of an SMP box prevents _ONLY_ the
        boot CPU apic timer from being used.  But the secondary CPU is still
        unconditionally setting up the APIC timer and uses the non
        calibrated variable calibration_result, which is of course 0, to
        setup the APIC timer.  Wreckage guaranteed."
    
    so we'll just have to wait for the x86 merge to hopefully fix this up
    for x86-64.
    Tested-and-requested-by: default avatarRafael J. Wysocki <rjw@sisk.pl>
    Acked-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    f7f847b0
apic.h 2.73 KB