x86: Add a synthetic TSC_RELIABLE feature bit.

Impact: Changes timebase calibration on Vmware.

Use the synthetic TSC_RELIABLE bit to workaround virtualization anomalies.

Virtual TSCs can be kept nearly in sync, but because the virtual TSC
offset is set by software, it's not perfect.  So, the TSC
synchronization test can fail. Even then the TSC can be used as a
clocksource since the VMware platform exports a reliable TSC to the
guest for timekeeping purposes. Use this bit to check if we need to
skip the TSC sync checks.

Along with this also set the CONSTANT_TSC bit when on VMware, since we
still want to use TSC as clocksource on VM running over hardware which
has unsynchronized TSC's (opteron's), since the hypervisor will take
care of providing consistent TSC to the guest.

Signed-off-by: Alok N Kataria <akataria@vmware.com>
Signed-off-by: Dan Hecht <dhecht@vmware.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
index 9ffb01c31c40..5977c40a138f 100644
--- a/arch/x86/kernel/tsc_sync.c
+++ b/arch/x86/kernel/tsc_sync.c
@@ -108,6 +108,12 @@ void __cpuinit check_tsc_sync_source(int cpu)
         if (unsynchronized_tsc())
                 return;
 
+        if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE)) {
+                printk(KERN_INFO
+                       "Skipping synchronization checks as TSC is reliable.\n");
+                return;
+        }
+
         printk(KERN_INFO "checking TSC synchronization [CPU#%d -> CPU#%d]:",
                           smp_processor_id(), cpu);
 
@@ -161,7 +167,7 @@ void __cpuinit check_tsc_sync_target(void)
 {
         int cpus = 2;
 
-        if (unsynchronized_tsc())
+        if (unsynchronized_tsc() || boot_cpu_has(X86_FEATURE_TSC_RELIABLE))
                 return;
 
         /*