Commit 46543ba4 authored by Zachary Amsden's avatar Zachary Amsden Committed by Avi Kivity
Browse files

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: default avatarZachary Amsden <zamsden@redhat.com>
Signed-off-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
parent 759379dd

arch/x86/kvm/x86.c

+10 −4
Original line number Diff line number Diff line
@@ -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);