KVM: x86: Keep the lock order consistent between SRCU and gpc spinlock

Acquire SRCU before taking the gpc spinlock in wait_pending_event() so as
to be consistent with all other functions that acquire both locks.  It's
not illegal to acquire SRCU inside a spinlock, nor is there deadlock
potential, but in general it's preferable to order locks from least
restrictive to most restrictive, e.g. if wait_pending_event() needed to
sleep for whatever reason, it could do so while holding SRCU, but would
need to drop the spinlock.
Signed-off-by: Peng Hao <flyingpeng@tencent.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/CAPm50a++Cb=QfnjMZ2EnCj-Sb9Y4UM-=uOEtHAcjnNLCAAf-dQ@mail.gmail.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/xen.c

@@ -1165,8 +1165,8 @@ static bool wait_pending_event(struct kvm_vcpu *vcpu, int nr_ports,
 	bool ret = true;
 	int idx, i;
 
-	read_lock_irqsave(&gpc->lock, flags);
 	idx = srcu_read_lock(&kvm->srcu);
+	read_lock_irqsave(&gpc->lock, flags);
 	if (!kvm_gpc_check(kvm, gpc, gpc->gpa, PAGE_SIZE))
 		goto out_rcu;
 
@@ -1187,8 +1187,8 @@ static bool wait_pending_event(struct kvm_vcpu *vcpu, int nr_ports,
 	}
 
  out_rcu:
-	srcu_read_unlock(&kvm->srcu, idx);
 	read_unlock_irqrestore(&gpc->lock, flags);
+	srcu_read_unlock(&kvm->srcu, idx);
 
 	return ret;
 }