[PATCH RFC: kvm tsc virtualization 08/20] Export the reference TSC from KVM module

[Zachary Amsden <zamsden@redhat.com>]

KVM now maintains a reference TSC, which can be exported to the
individual backends for private use.

Signed-off-by: Zachary Amsden <zamsden@redhat.com>
---
 arch/x86/kvm/x86.c |   25 +++++++++++++++++++++++++
 1 files changed, 25 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 144a820..170fd5d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -742,6 +742,7 @@ static DEFINE_PER_CPU(int, cpu_tsc_shift);
 static DEFINE_PER_CPU(s64, cpu_tsc_offset);
 static DEFINE_PER_CPU(u64, cpu_tsc_measure_base);
 static DEFINE_PER_CPU(atomic_t, cpu_tsc_synchronized);
+static DEFINE_PER_CPU(int, cpu_tsc_generation);
 static int tsc_base_cpu = -1;
 static unsigned long ref_tsc_khz;
 
@@ -813,6 +814,29 @@ static inline u64 compute_ref_tsc(int cpu)
 	return tsc + per_cpu(cpu_tsc_offset, cpu);
 }
 
+/*
+ * kvm_get_ref_tsc returns a reference TSC value which we attempt to keep
+ * stable across all CPUs, regardless of frequency shift, CPU hotplug, or
+ * pre-existing TSC offsets.
+ */
+u64 kvm_get_ref_tsc(void)
+{
+	int cpu, gen;
+	u64 tsc;
+
+	cpu = get_cpu();
+again:
+	gen = per_cpu(cpu_tsc_generation, cpu);
+	smp_rmb();
+	tsc = compute_ref_tsc(cpu);
+	smp_rmb();
+	if (unlikely(gen != per_cpu(cpu_tsc_generation, cpu)))
+		goto again;
+	put_cpu();
+	return tsc;
+}
+EXPORT_SYMBOL_GPL(kvm_get_ref_tsc);
+
 #define SYNC_TRIES 64
 
 /*
@@ -929,6 +953,7 @@ static void kvm_sync_tsc(void *cpup)
 			accumulator -= delta[i+SYNC_TRIES];
 		accumulator = accumulator / (SYNC_TRIES*2-12);
 		per_cpu(cpu_tsc_offset, new_cpu) = accumulator;
+		++per_cpu(cpu_tsc_generation, new_cpu);
 		atomic_set(&per_cpu(cpu_tsc_synchronized, new_cpu), 1);
 		pr_debug("%s: OUT, cpu = %d, cpu_tsc_offset = %lld, cpu_tsc_multiplier=%ld, cpu_tsc_shift=%d\n", __func__, raw_smp_processor_id(), per_cpu(cpu_tsc_offset, new_cpu), per_cpu(cpu_tsc_multiplier, new_cpu), per_cpu(cpu_tsc_shift, new_cpu));
 	}
-- 
1.6.5.2