KVM: x86: TSC reset compensation

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>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 30 insertions, 1 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 886132b6ef14..e7da14c317e6 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -898,11 +898,40 @@ static DEFINE_PER_CPU(unsigned long, cpu_tsc_khz);
 void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
 {
         struct kvm *kvm = vcpu->kvm;
-        u64 offset;
+        u64 offset, ns, elapsed;
         unsigned long flags;
+        struct timespec ts;
 
         spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
         offset = data - native_read_tsc();
+        ktime_get_ts(&ts);
+        monotonic_to_bootbased(&ts);
+        ns = timespec_to_ns(&ts);
+        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 < 5ULL * NSEC_PER_SEC) {
+                if (!check_tsc_unstable()) {
+                        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);
         spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);