x86: support always running TSC on Intel CPUs

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: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 1 insertions, 1 deletions

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index c622772744d..18c70fedba3 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -270,7 +270,7 @@ static void c1e_idle(void)
                 rdmsr(MSR_K8_INT_PENDING_MSG, lo, hi);
                 if (lo & K8_INTP_C1E_ACTIVE_MASK) {
                         c1e_detected = 1;
-                        if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
+                        if (!boot_cpu_has(X86_FEATURE_NONSTOP_TSC))
                                 mark_tsc_unstable("TSC halt in AMD C1E");
                         printk(KERN_INFO "System has AMD C1E enabled\n");
                         set_cpu_cap(&boot_cpu_data, X86_FEATURE_AMDC1E);