KVM: x86: Ensure PV TLB flush tracepoint reflects KVM behavior

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>
Message-Id: <20210531174628.10265-1-jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1cd6d46..e2144ee 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3101,9 +3101,11 @@ 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;