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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>
This commit is contained in:
Zachary Amsden 2010-08-19 22:07:26 -10:00 committed by Avi Kivity
parent 759379dd68
commit 46543ba45f
1 changed files with 10 additions and 4 deletions
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14
arch/x86/kvm/x86.c
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@ -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);