[PATCH 16/17] TSC reset compensation

Zachary Amsden
2010-06-15T07:35:07Z

Attempt to synchronize TSCs which are reset to the same value. In the
case of a reliable hardware TSC, we can just re-use the same offset, but
on non-reliable hardware, we can get closer by adjusting the offset to
match the elapsed time.

Signed-off-by: Zachary Amsden <zamsden@redhat.com>
---
arch/x86/kvm/x86.c | 34 ++++++++++++++++++++++++++++++++--
1 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8e836e9..cedb71f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -937,14 +937,44 @@ static inline void kvm_request_guest_time_update(struct kvm_vcpu *v)
set_bit(KVM_REQ_CLOCK_SYNC, &v->requests);
}

+static inline int kvm_tsc_reliable(void)
+{
+ return (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
+ boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
+ !check_tsc_unstable());
+}
+
void guest_write_tsc(struct kvm_vcpu *vcpu, u64 data)
{
struct kvm *kvm = vcpu->kvm;
- u64 offset;
+ u64 offset, ns, elapsed;

spin_lock(&kvm->arch.tsc_write_lock);
offset = data - native_read_tsc();
- kvm->arch.last_tsc_nsec = get_kernel_ns();
+ ns = get_kernel_ns();
+ elapsed = ns - kvm->arch.last_tsc_nsec;
+
+ /*
+ * Special case: identical write to TSC within 5 seconds of
+ * another CPU is interpreted as an attempt to synchronize
+ * (the 5 seconds is to accomodate host load / swapping).
+ *
+ * In that case, for a reliable TSC, we can match TSC offsets,
+ * or make a best guest using kernel_ns value.
+ */
+ if (data == kvm->arch.last_tsc_write && elapsed < 5 * NSEC_PER_SEC) {
+ if (kvm_tsc_reliable()) {
+ offset = kvm->arch.last_tsc_offset;
+ pr_debug("kvm: matched tsc offset for %llu\n", data);
+ } else {
+ u64 tsc_delta = elapsed * __get_cpu_var(cpu_tsc_khz);
+ tsc_delta = tsc_delta / USEC_PER_SEC;
+ offset -= tsc_delta;
+ pr_debug("kvm: adjusted tsc offset by %llu\n", tsc_delta);
+ }
+ ns = kvm->arch.last_tsc_nsec;
+ }
+ kvm->arch.last_tsc_nsec = ns;
kvm->arch.last_tsc_write = data;
kvm->arch.last_tsc_offset = offset;
kvm_x86_ops->write_tsc_offset(vcpu, offset);
--
1.7.1