KVM: Add last_host_tsc tracking back to KVM

The variable last_host_tsc was removed from upstream code.  I am adding
it back for two reasons.  First, it is unnecessary to use guest TSC
computation to conclude information about the host TSC.  The guest may
set the TSC backwards (this case handled by the previous patch), but
the computation of guest TSC (and fetching an MSR) is significanlty more
work and complexity than simply reading the hardware counter.  In addition,
we don't actually need the guest TSC for any part of the computation,
by always recomputing the offset, we can eliminate the need to deal with
the current offset and any scaling factors that may apply.

The second reason is that later on, we are going to be using the host
TSC value to restore TSC offsets after a host S4 suspend, so we need to
be reading the host values, not the guest values here.

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

1 files changed, 3 insertions, 8 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2a59f76d96f1..39a57dac884a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2253,13 +2253,8 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 
         kvm_x86_ops->vcpu_load(vcpu, cpu);
         if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) {
-                /* Make sure TSC doesn't go backwards */
-                s64 tsc_delta;
-                u64 tsc;
-
-                tsc = kvm_x86_ops->read_l1_tsc(vcpu);
-                tsc_delta = tsc - vcpu->arch.last_guest_tsc;
-
+                s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
+                                native_read_tsc() - vcpu->arch.last_host_tsc;
                 if (tsc_delta < 0)
                         mark_tsc_unstable("KVM discovered backwards TSC");
                 if (check_tsc_unstable()) {
@@ -2282,7 +2277,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 {
         kvm_x86_ops->vcpu_put(vcpu);
         kvm_put_guest_fpu(vcpu);
-        vcpu->arch.last_guest_tsc = kvm_x86_ops->read_l1_tsc(vcpu);
+        vcpu->arch.last_host_tsc = native_read_tsc();
 }
 
 static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu,