• Venki Pallipadi's avatar
    x86: support always running TSC on Intel CPUs · 40fb1715
    Venki Pallipadi authored
    Impact: reward non-stop TSCs with good TSC-based clocksources, etc.
    
    Add support for CPUID_0x80000007_Bit8 on Intel CPUs as well. This bit means
    that the TSC is invariant with C/P/T states and always runs at constant
    frequency.
    
    With Intel CPUs, we have 3 classes
    * CPUs where TSC runs at constant rate and does not stop n C-states
    * CPUs where TSC runs at constant rate, but will stop in deep C-states
    * CPUs where TSC rate will vary based on P/T-states and TSC will stop in deep
      C-states.
    
    To cover these 3, one feature bit (CONSTANT_TSC) is not enough. So, add a
    second bit (NONSTOP_TSC). CONSTANT_TSC indicates that the TSC runs at
    constant frequency irrespective of P/T-states, and NONSTOP_TSC indicates
    that TSC does not stop in deep C-states.
    
    CPUID_0x8000000_Bit8 indicates both these feature bit can be set.
    We still have CONSTANT_TSC _set_ and NONSTOP_TSC _not_set_ on some older Intel
    CPUs, based on model checks. We can use TSC on such CPUs for time, as long as
    those CPUs do not support/enter deep C-states.
    Signed-off-by: default avatarVenkatesh Pallipadi <venkatesh.pallipadi@intel.com>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
    40fb1715
amd.c 11.3 KB