• Thomas Gleixner's avatar
    x86/tsc: Store and check TSC ADJUST MSR · 8b223bc7
    Thomas Gleixner authored
    The TSC_ADJUST MSR shows whether the TSC has been modified. This is helpful
    in a two aspects:
    
    1) It allows to detect BIOS wreckage, where SMM code tries to 'hide' the
       cycles spent by storing the TSC value at SMM entry and restoring it at
       SMM exit. On affected machines the TSCs run slowly out of sync up to the
       point where the clocksource watchdog (if available) detects it.
    
       The TSC_ADJUST MSR allows to detect the TSC modification before that and
       eventually restore it. This is also important for SoCs which have no
       watchdog clocksource and therefore TSC wreckage cannot be detected and
       acted upon.
    
    2) All threads in a package are required to have the same TSC_ADJUST
       value. Broken BIOSes break that and as a result the TSC synchronization
       check fails.
    
       The TSC_ADJUST MSR allows to detect the deviation when a CPU comes
       online. If detected set it to the value of an already online CPU in the
       same package. This also allows to reduce the number of sync tests
       because with that in place the test is only required for the first CPU
       in a package.
    
       In principle all CPUs in a system should have the same TSC_ADJUST value
       even across packages, but with physical CPU hotplug this assumption is
       not true because the TSC starts with power on, so physical hotplug has
       to do some trickery to bring the TSC into sync with already running
       packages, which requires to use an TSC_ADJUST value different from CPUs
       which got powered earlier.
    
       A final enhancement is the opportunity to compensate for unsynced TSCs
       accross nodes at boot time and make the TSC usable that way. It won't
       help for TSCs which run apart due to frequency skew between packages,
       but this gets detected by the clocksource watchdog later.
    
    The first step toward this is to store the TSC_ADJUST value of a starting
    CPU and compare it with the value of an already online CPU in the same
    package. If they differ, emit a warning and adjust it to the reference
    value. The !SMP version just stores the boot value for later verification.
    Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Reviewed-by: default avatarIngo Molnar <mingo@kernel.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Yinghai Lu <yinghai@kernel.org>
    Cc: Borislav Petkov <bp@alien8.de>
    Link: http://lkml.kernel.org/r/20161119134017.655323776@linutronix.deSigned-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    8b223bc7
Makefile 4.38 KB