KVM: x86: Robust TSC compensation

Make the match of TSC find TSC writes that are close to each other
instead of perfectly identical; this allows the compensator to also
work in migration / suspend scenarios.

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

1 files changed, 10 insertions, 4 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4bcb120cc76a..4ff0c271f125 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -928,21 +928,27 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
         struct kvm *kvm = vcpu->kvm;
         u64 offset, ns, elapsed;
         unsigned long flags;
+        s64 sdiff;
 
         spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
         offset = data - native_read_tsc();
         ns = get_kernel_ns();
         elapsed = ns - kvm->arch.last_tsc_nsec;
+        sdiff = data - kvm->arch.last_tsc_write;
+        if (sdiff < 0)
+                sdiff = -sdiff;
 
         /*
-         * Special case: identical write to TSC within 5 seconds of
+         * Special case: close 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).
+         * The 5 seconds is to accomodate host load / swapping as
+         * well as any reset of TSC during the boot process.
          *
          * In that case, for a reliable TSC, we can match TSC offsets,
-         * or make a best guest using kernel_ns value.
+         * or make a best guest using elapsed value.
          */
-        if (data == kvm->arch.last_tsc_write && elapsed < 5ULL * NSEC_PER_SEC) {
+        if (sdiff < nsec_to_cycles(5ULL * NSEC_PER_SEC) &&
+            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);