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Subject[PATCH] Skip tsc synchronization checks if CONSTANT_TSC bit is set.
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Skip tsc synchronization checks if CONSTANT_TSC bit is set.

From: Alok N Kataria <akataria@vmware.com>

TSC synchronization checks between CPU's bail out even if we see
a distortion of a single cycle. This makes the TSC mostly unsuable
in a virtualized environment.

The CONSTANT_TSC bit tells us if the hardware exports a constant TSC,
we can use this bit to trust the hardware and skip the TSC sync checks
at bootup.

We also force set the CONSTANT_TSC capability bit if we are running on
VMware, since the VMware hypervisor exports a constant TSC to the guest.

Signed-off-by: Alok N Kataria <akataria@vmware.com>
Cc: Andi Kleen <andi@firstfloor.org>
---

arch/x86/kernel/cpu/common.c | 1 +
arch/x86/kernel/tsc_sync.c | 8 +++++++-
2 files changed, 8 insertions(+), 1 deletions(-)


diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index a48cb0e..720b583 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -452,6 +452,7 @@ void __cpuinit detect_hypervisor_vendor(struct cpuinfo_x86 *c)
{
if (vmware_platform()) {
c->x86_hyper_vendor = X86_HYPER_VENDOR_VMWARE;
+ set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
} else {
c->x86_hyper_vendor = X86_HYPER_VENDOR_NONE;
}
diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
index 9ffb01c..c0c89b7 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_CONSTANT_TSC)) {
+ printk(KERN_INFO
+ "Skipping synchronization checks as TSC is constant.\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_CONSTANT_TSC))
return;

/*



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