[PATCH 09/18] Robust TSC compensation

Zachary Amsden
2010-07-13T02:25:29Z

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>
---
arch/x86/kvm/x86.c | 14 ++++++++++----
1 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 79c4608..51d3f3e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -926,21 +926,27 @@ void guest_write_tsc(struct kvm_vcpu *vcpu, u64 data)
struct kvm *kvm = vcpu->kvm;
u64 offset, ns, elapsed;
struct timespec ts;
+ s64 sdiff;

spin_lock(&kvm->arch.tsc_write_lock);
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);
--
1.7.1