From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [RFC v4 3/5] virtio_ring: add packed ring support Date: Wed, 16 May 2018 22:05:44 +0800 Message-ID: References: <20180516083737.26504-1-tiwei.bie@intel.com> <20180516083737.26504-4-tiwei.bie@intel.com> <2000f635-bc34-71ff-ff51-a711c2e9726d@redhat.com> <20180516123909.GB986@debian> <20180516134550.GB4171@debian> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Cc: mst@redhat.com, virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, wexu@redhat.com, jfreimann@redhat.com To: Tiwei Bie Return-path: In-Reply-To: <20180516134550.GB4171@debian> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 2018年05月16日 21:45, Tiwei Bie wrote: > On Wed, May 16, 2018 at 08:51:43PM +0800, Jason Wang wrote: >> On 2018年05月16日 20:39, Tiwei Bie wrote: >>> On Wed, May 16, 2018 at 07:50:16PM +0800, Jason Wang wrote: >>>> On 2018年05月16日 16:37, Tiwei Bie wrote: >>> [...] >>>>> struct vring_virtqueue { >>>>> @@ -116,6 +117,9 @@ struct vring_virtqueue { >>>>> /* Last written value to driver->flags in >>>>> * guest byte order. */ >>>>> u16 event_flags_shadow; >>>>> + >>>>> + /* ID allocation. */ >>>>> + struct idr buffer_id; >>>> I'm not sure idr is fit for the performance critical case here. Need to >>>> measure its performance impact, especially if we have few unused slots. >>> I'm also not sure.. But fortunately, it should be quite easy >>> to replace it with something else without changing other code. >>> If it will really hurt the performance, I'll change it. >> We may want to do some benchmarking/profiling to see. > Yeah! > >>>>> }; >>>>> }; >>> [...] >>>>> +static void detach_buf_packed(struct vring_virtqueue *vq, unsigned int head, >>>>> + unsigned int id, void **ctx) >>>>> +{ >>>>> + struct vring_packed_desc *desc; >>>>> + unsigned int i, j; >>>>> + >>>>> + /* Clear data ptr. */ >>>>> + vq->desc_state[id].data = NULL; >>>>> + >>>>> + i = head; >>>>> + >>>>> + for (j = 0; j < vq->desc_state[id].num; j++) { >>>>> + desc = &vq->vring_packed.desc[i]; >>>>> + vring_unmap_one_packed(vq, desc); >>>> As mentioned in previous discussion, this probably won't work for the case >>>> of out of order completion since it depends on the information in the >>>> descriptor ring. We probably need to extend ctx to record such information. >>> Above code doesn't depend on the information in the descriptor >>> ring. The vq->desc_state[] is the extended ctx. >>> >>> Best regards, >>> Tiwei Bie >> Yes, but desc is a pointer to descriptor ring I think so >> vring_unmap_one_packed() still depends on the content of descriptor ring? >> > I got your point now. I think it makes sense to reserve > the bits of the addr field. Driver shouldn't try to get > addrs from the descriptors when cleanup the descriptors > no matter whether we support out-of-order or not. Maybe I was wrong, but I remember spec mentioned something like this. > > But combining it with the out-of-order support, it will > mean that the driver still needs to maintain a desc/ctx > list that is very similar to the desc ring in the split > ring. I'm not quite sure whether it's something we want. > If it is true, I'll do it. So do you think we also want > to maintain such a desc/ctx list for packed ring? To make it work for OOO backends I think we need something like this (hardware NIC drivers are usually have something like this). Not for the patch, but it looks like having a OUT_OF_ORDER feature bit is much more simpler to be started with. Thanks > > Best regards, > Tiwei Bie