[PATCH 1/2] KVM: lapic: fix preemption timer backward when TSC backward

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpeng.li@hotmail.com>

TSC_OFFSET will be adjusted if discovers TSC backward during vCPU load. 
The preemption timer which will leverage guest tsc to reprogram its 
preemption timer value is also reprogrammed if vCPU is scheded in to 
a different pCPU. However, current implementation reprogram preemption 
timer before TSC_OFFSET is adjusted to the right value, this will result 
in preemption timer also backward and fire prematurity.

This patch fix it by adjusting TSC_OFFSET before reprogramming preemption 
timer if TSC backward.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Yunhong Jiang <yunhong.jiang@intel.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
 arch/x86/kvm/x86.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 19f9f9e..699f872 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2743,16 +2743,16 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 		if (tsc_delta < 0)
 			mark_tsc_unstable("KVM discovered backwards TSC");
 
-		if (kvm_lapic_hv_timer_in_use(vcpu) &&
-				kvm_x86_ops->set_hv_timer(vcpu,
-					kvm_get_lapic_tscdeadline_msr(vcpu)))
-			kvm_lapic_switch_to_sw_timer(vcpu);
 		if (check_tsc_unstable()) {
 			u64 offset = kvm_compute_tsc_offset(vcpu,
 						vcpu->arch.last_guest_tsc);
 			kvm_x86_ops->write_tsc_offset(vcpu, offset);
 			vcpu->arch.tsc_catchup = 1;
 		}
+		if (kvm_lapic_hv_timer_in_use(vcpu) &&
+				kvm_x86_ops->set_hv_timer(vcpu,
+					kvm_get_lapic_tscdeadline_msr(vcpu)))
+			kvm_lapic_switch_to_sw_timer(vcpu);
 		/*
 		 * On a host with synchronized TSC, there is no need to update
 		 * kvmclock on vcpu->cpu migration
-- 
1.9.1