From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sheng Yang Subject: Re: [PATCH 4/6] KVM: Add kvm_get_irq_routing_entry() func Date: Thu, 18 Nov 2010 10:22:15 +0800 Message-ID: <201011181022.15853.sheng@linux.intel.com> References: <1289812532-3227-1-git-send-email-sheng@linux.intel.com> <1289812532-3227-5-git-send-email-sheng@linux.intel.com> <4CE3E045.30500@redhat.com> Mime-Version: 1.0 Content-Type: Text/Plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , "Michael S. Tsirkin" , kvm@vger.kernel.org To: Avi Kivity Return-path: Received: from mga02.intel.com ([134.134.136.20]:41670 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754533Ab0KRCWS (ORCPT ); Wed, 17 Nov 2010 21:22:18 -0500 In-Reply-To: <4CE3E045.30500@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Wednesday 17 November 2010 22:01:41 Avi Kivity wrote: > On 11/15/2010 11:15 AM, Sheng Yang wrote: > > We need to query the entry later. > > > > +int kvm_get_irq_routing_entry(struct kvm *kvm, int gsi, > > + struct kvm_kernel_irq_routing_entry *entry) > > +{ > > + int count = 0; > > + struct kvm_kernel_irq_routing_entry *ei = NULL; > > + struct kvm_irq_routing_table *irq_rt; > > + struct hlist_node *n; > > + > > + rcu_read_lock(); > > + irq_rt = rcu_dereference(kvm->irq_routing); > > + if (gsi< irq_rt->nr_rt_entries) > > + hlist_for_each_entry(ei, n,&irq_rt->map[gsi], link) > > + count++; > > + if (count == 1) > > + *entry = *ei; > > + rcu_read_unlock(); > > + > > + return (count != 1); > > +} > > + > > Not good form to rely on ei being valid after the loop. > > I guess this is only useful for msi? Need to document it. May can be used for others later, it's somehow generic. Where should I document it? > > *entry may be stale after rcu_read_unlock(). Is this a problem? I suppose not. All MSI-X MMIO accessing would be executed without delay, so no re- order issue would happen. If the guest is reading and writing the field at the same time(from two cpus), it should got some kinds of sync method for itself - or it may not care what's the reading result(like the one after msix_mask_irq()). -- regards Yang, Sheng