[PATCH] KVM: X86: reset and read st->preempted in atomic way

[Lai Jiangshan <jiangshanlai@gmail.com>]

From: Lai Jiangshan <laijs@linux.alibaba.com>

In record_steal_time(), st->preempted is read twice, and
trace_kvm_pv_tlb_flush() might output result inconsistent if
kvm_vcpu_flush_tlb_guest() see a different st->preempted later.

It is a very trivial problem and hardly has actual harm and can be
avoided by reseting and reading st->preempted in atomic way via xchg().

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0087d3532c98..fba39fe162da 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3117,9 +3117,11 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
 	 * expensive IPIs.
 	 */
 	if (guest_pv_has(vcpu, KVM_FEATURE_PV_TLB_FLUSH)) {
+		u8 st_preempted = xchg(&st->preempted, 0);
+
 		trace_kvm_pv_tlb_flush(vcpu->vcpu_id,
-				       st->preempted & KVM_VCPU_FLUSH_TLB);
-		if (xchg(&st->preempted, 0) & KVM_VCPU_FLUSH_TLB)
+				       st_preempted & KVM_VCPU_FLUSH_TLB);
+		if (st_preempted & KVM_VCPU_FLUSH_TLB)
 			kvm_vcpu_flush_tlb_guest(vcpu);
 	} else {
 		st->preempted = 0;
-- 
2.19.1.6.gb485710b