From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tiwei Bie Subject: Re: [RFC v6 4/5] virtio_ring: add event idx support in packed ring Date: Fri, 8 Jun 2018 16:32:13 +0800 Message-ID: <20180608083213.GA8512@debian> References: <20180605074046.20709-1-tiwei.bie@intel.com> <20180605074046.20709-5-tiwei.bie@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 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: Jason Wang Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Thu, Jun 07, 2018 at 05:50:32PM +0800, Jason Wang wrote: > On 2018年06月05日 15:40, Tiwei Bie wrote: > > static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq) > > { > > struct vring_virtqueue *vq = to_vvq(_vq); > > + u16 bufs, used_idx, wrap_counter; > > START_USE(vq); > > /* We optimistically turn back on interrupts, then check if there was > > * more to do. */ > > + /* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to > > + * either clear the flags bit or point the event index at the next > > + * entry. Always update the event index to keep code simple. */ > > + > > Maybe for packed ring, it's time to treat event index separately to avoid a > virtio_wmb() for event idx is off. Okay. I'll do it. > > > + /* TODO: tune this threshold */ > > + if (vq->next_avail_idx < vq->last_used_idx) > > + bufs = (vq->vring_packed.num + vq->next_avail_idx - > > + vq->last_used_idx) * 3 / 4; > > + else > > + bufs = (vq->next_avail_idx - vq->last_used_idx) * 3 / 4; > > vq->next_avail-idx could be equal to vq->last_usd_idx when the ring is full. > Though virito-net is the only user now and it can guarantee this won't > happen. But consider this is a core API, we should make sure it can work for > any cases. > > It looks to me that bufs is just vq->vring_packed.num - vq->num_free? I'll try it! Thanks for the suggestion! :) > > > + > > + wrap_counter = vq->used_wrap_counter; > > + > > + used_idx = vq->last_used_idx + bufs; > > + if (used_idx >= vq->vring_packed.num) { > > + used_idx -= vq->vring_packed.num; > > + wrap_counter ^= 1; > > + } > > + > > + vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev, > > + used_idx | (wrap_counter << 15)); > > if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) { > > - vq->event_flags_shadow = VRING_EVENT_F_ENABLE; > > + /* We need to update event offset and event wrap > > + * counter first before updating event flags. */ > > + virtio_wmb(vq->weak_barriers); > > + vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC : > > + VRING_EVENT_F_ENABLE; > > vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev, > > vq->event_flags_shadow); > > - /* We need to enable interrupts first before re-checking > > - * for more used buffers. */ > > - virtio_mb(vq->weak_barriers); > > } > > + /* We need to update event suppression structure first > > + * before re-checking for more used buffers. */ > > + virtio_mb(vq->weak_barriers); > > + > > if (more_used_packed(vq)) { > > END_USE(vq); > > return false; > > I think what we need to to make sure the descriptor used_idx is used? > Otherwise we may stop and restart qdisc too frequently? Good catch! Yeah. It's not the best choice to check the descriptor at last_used_idx. I'll try to fix this! Best regards, Tiwei Bie > > Thanks > > > -- >