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>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4bcb120cc76adcff7c453117e73c6338276496bb..4ff0c271f1259c4fb2cd8e3ad269caea7cde52c6 100644 (file)
--- 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);