Re: [PATCH v9 31/32] virtio_net: support rx/tx queue resize

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Wed, 13 Apr 2022 16:00:18 +0800, Jason Wang <jasowang@redhat.com> wrote:
>
> 在 2022/4/6 上午11:43, Xuan Zhuo 写道:
> > This patch implements the resize function of the rx, tx queues.
> > Based on this function, it is possible to modify the ring num of the
> > queue.
> >
> > There may be an exception during the resize process, the resize may
> > fail, or the vq can no longer be used. Either way, we must execute
> > napi_enable(). Because napi_disable is similar to a lock, napi_enable
> > must be called after calling napi_disable.
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > ---
> >   drivers/net/virtio_net.c | 81 ++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 81 insertions(+)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index b8bf00525177..ba6859f305f7 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -251,6 +251,9 @@ struct padded_vnet_hdr {
> >   	char padding[4];
> >   };
> >
> > +static void virtnet_sq_free_unused_buf(struct virtqueue *vq, void *buf);
> > +static void virtnet_rq_free_unused_buf(struct virtqueue *vq, void *buf);
> > +
> >   static bool is_xdp_frame(void *ptr)
> >   {
> >   	return (unsigned long)ptr & VIRTIO_XDP_FLAG;
> > @@ -1369,6 +1372,15 @@ static void virtnet_napi_enable(struct virtqueue *vq, struct napi_struct *napi)
> >   {
> >   	napi_enable(napi);
> >
> > +	/* Check if vq is in reset state. The normal reset/resize process will
> > +	 * be protected by napi. However, the protection of napi is only enabled
> > +	 * during the operation, and the protection of napi will end after the
> > +	 * operation is completed. If re-enable fails during the process, vq
> > +	 * will remain unavailable with reset state.
> > +	 */
> > +	if (vq->reset)
> > +		return;
>
>
> I don't get when could we hit this condition.


In patch 23, the code to implement re-enable vq is as follows:

+static int vp_modern_enable_reset_vq(struct virtqueue *vq)
+{
+	struct virtio_pci_device *vp_dev = to_vp_device(vq->vdev);
+	struct virtio_pci_modern_device *mdev = &vp_dev->mdev;
+	struct virtio_pci_vq_info *info;
+	unsigned long flags, index;
+	int err;
+
+	if (!vq->reset)
+		return -EBUSY;
+
+	index = vq->index;
+	info = vp_dev->vqs[index];
+
+	/* check queue reset status */
+	if (vp_modern_get_queue_reset(mdev, index) != 1)
+		return -EBUSY;
+
+	err = vp_active_vq(vq, info->msix_vector);
+	if (err)
+		return err;
+
+	if (vq->callback) {
+		spin_lock_irqsave(&vp_dev->lock, flags);
+		list_add(&info->node, &vp_dev->virtqueues);
+		spin_unlock_irqrestore(&vp_dev->lock, flags);
+	} else {
+		INIT_LIST_HEAD(&info->node);
+	}
+
+	vp_modern_set_queue_enable(&vp_dev->mdev, index, true);
+
+	if (vp_dev->per_vq_vectors && info->msix_vector != VIRTIO_MSI_NO_VECTOR)
+		enable_irq(pci_irq_vector(vp_dev->pci_dev, info->msix_vector));
+
+	vq->reset = false;
+
+	return 0;
+}


There are three situations where an error will be returned. These are the
situations I want to handle.

But I'm rethinking the question, and I feel like you're right, although the
hardware setup may fail. We can no longer sync with the hardware. But using it
as a normal vq doesn't have any problems.

>
>
> > +
> >   	/* If all buffers were filled by other side before we napi_enabled, we
> >   	 * won't get another interrupt, so process any outstanding packets now.
> >   	 * Call local_bh_enable after to trigger softIRQ processing.
> > @@ -1413,6 +1425,15 @@ static void refill_work(struct work_struct *work)
> >   		struct receive_queue *rq = &vi->rq[i];
> >
> >   		napi_disable(&rq->napi);
> > +
> > +		/* Check if vq is in reset state. See more in
> > +		 * virtnet_napi_enable()
> > +		 */
> > +		if (rq->vq->reset) {
> > +			virtnet_napi_enable(rq->vq, &rq->napi);
> > +			continue;
> > +		}
>
>
> Can we do something similar in virtnet_close() by canceling the work?

I think there is no need to cancel the work here, because napi_disable will wait
for the napi_enable of the resize. So if the re-enable failed vq is used as a normal
vq, this logic can be removed.


>
>
> > +
> >   		still_empty = !try_fill_recv(vi, rq, GFP_KERNEL);
> >   		virtnet_napi_enable(rq->vq, &rq->napi);
> >
> > @@ -1523,6 +1544,10 @@ static void virtnet_poll_cleantx(struct receive_queue *rq)
> >   	if (!sq->napi.weight || is_xdp_raw_buffer_queue(vi, index))
> >   		return;
> >
> > +	/* Check if vq is in reset state. See more in virtnet_napi_enable() */
> > +	if (sq->vq->reset)
> > +		return;
>
>
> We've disabled TX napi, any chance we can still hit this?

Same as above.

>
>
> > +
> >   	if (__netif_tx_trylock(txq)) {
> >   		do {
> >   			virtqueue_disable_cb(sq->vq);
> > @@ -1769,6 +1794,62 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> >   	return NETDEV_TX_OK;
> >   }
> >
> > +static int virtnet_rx_resize(struct virtnet_info *vi,
> > +			     struct receive_queue *rq, u32 ring_num)
> > +{
> > +	int err;
> > +
> > +	napi_disable(&rq->napi);
> > +
> > +	err = virtqueue_resize(rq->vq, ring_num, virtnet_rq_free_unused_buf);
> > +	if (err)
> > +		goto err;
> > +
> > +	if (!try_fill_recv(vi, rq, GFP_KERNEL))
> > +		schedule_delayed_work(&vi->refill, 0);
> > +
> > +	virtnet_napi_enable(rq->vq, &rq->napi);
> > +	return 0;
> > +
> > +err:
> > +	netdev_err(vi->dev,
> > +		   "reset rx reset vq fail: rx queue index: %td err: %d\n",
> > +		   rq - vi->rq, err);
> > +	virtnet_napi_enable(rq->vq, &rq->napi);
> > +	return err;
> > +}
> > +
> > +static int virtnet_tx_resize(struct virtnet_info *vi,
> > +			     struct send_queue *sq, u32 ring_num)
> > +{
> > +	struct netdev_queue *txq;
> > +	int err, qindex;
> > +
> > +	qindex = sq - vi->sq;
> > +
> > +	virtnet_napi_tx_disable(&sq->napi);
> > +
> > +	txq = netdev_get_tx_queue(vi->dev, qindex);
> > +	__netif_tx_lock_bh(txq);
> > +	netif_stop_subqueue(vi->dev, qindex);
> > +	__netif_tx_unlock_bh(txq);
> > +
> > +	err = virtqueue_resize(sq->vq, ring_num, virtnet_sq_free_unused_buf);
> > +	if (err)
> > +		goto err;
> > +
> > +	netif_start_subqueue(vi->dev, qindex);
> > +	virtnet_napi_tx_enable(vi, sq->vq, &sq->napi);
> > +	return 0;
> > +
> > +err:
>
>
> I guess we can still start the queue in this case? (Since we don't
> change the queue if resize fails).

Yes, you are right.

Thanks.

>
>
> > +	netdev_err(vi->dev,
> > +		   "reset tx reset vq fail: tx queue index: %td err: %d\n",
> > +		   sq - vi->sq, err);
> > +	virtnet_napi_tx_enable(vi, sq->vq, &sq->napi);
> > +	return err;
> > +}
> > +
> >   /*
> >    * Send command via the control virtqueue and check status.  Commands
> >    * supported by the hypervisor, as indicated by feature bits, should
>