netdev - Re: [PATCH 2/4] [RFC rev2] virtio-net changes

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Message-ID: <878vvf0wkd.fsf@rustcorp.com.au>
Date:	Wed, 13 Apr 2011 10:58:02 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Krishna Kumar <krkumar2@...ibm.com>, davem@...emloft.net,
	mst@...hat.com
Cc:	eric.dumazet@...il.com, arnd@...db.de, netdev@...r.kernel.org,
	horms@...ge.net.au, avi@...hat.com, anthony@...emonkey.ws,
	kvm@...r.kernel.org, Krishna Kumar <krkumar2@...ibm.com>
Subject: Re: [PATCH 2/4] [RFC rev2] virtio-net changes

On Tue, 05 Apr 2011 20:38:52 +0530, Krishna Kumar <krkumar2@...ibm.com> wrote:
> Implement mq virtio-net driver. 
> 
> Though struct virtio_net_config changes, it works with the old
> qemu since the last element is not accessed unless qemu sets
> VIRTIO_NET_F_MULTIQUEUE.
> 
> Signed-off-by: Krishna Kumar <krkumar2@...ibm.com>

Hi Krishna!

    This change looks fairly solid, but I'd prefer it split into a few
stages for clarity.

The first patch should extract out the struct send_queue and struct
receive_queue, even though there's still only one.  The second patch
can then introduce VIRTIO_NET_F_MULTIQUEUE.

You could split into more parts if that makes sense, but I'd prefer to
see the mechanical changes separate from the feature addition.

> -struct virtnet_info {
> -	struct virtio_device *vdev;
> -	struct virtqueue *rvq, *svq, *cvq;
> -	struct net_device *dev;
> +/* Internal representation of a send virtqueue */
> +struct send_queue {
> +	/* Virtqueue associated with this send _queue */
> +	struct virtqueue *svq;

You can simply call this vq now it's inside 'send_queue'.

> +
> +	/* TX: fragments + linear part + virtio header */
> +	struct scatterlist tx_sg[MAX_SKB_FRAGS + 2];

Similarly, this can just be sg.

> +static void free_receive_bufs(struct virtnet_info *vi)
> +{
> +	int i;
> +
> +	for (i = 0; i < vi->numtxqs; i++) {
> +		BUG_ON(vi->rq[i] == NULL);
> +		while (vi->rq[i]->pages)
> +			__free_pages(get_a_page(vi->rq[i], GFP_KERNEL), 0);
> +	}
> +}

You can skip the BUG_ON(), since the next line will have the same effect.

> +/* Free memory allocated for send and receive queues */
> +static void free_rq_sq(struct virtnet_info *vi)
> +{
> +	int i;
> +
> +	if (vi->rq) {
> +		for (i = 0; i < vi->numtxqs; i++)
> +			kfree(vi->rq[i]);
> +		kfree(vi->rq);
> +	}
> +
> +	if (vi->sq) {
> +		for (i = 0; i < vi->numtxqs; i++)
> +			kfree(vi->sq[i]);
> +		kfree(vi->sq);
> +	}

This looks weird, even though it's correct.

I think we need a better name than numtxqs and shorter than
num_queue_pairs.  Let's just use num_queues; sure, there are both tx and
rq queues, but I still think it's pretty clear.

> +	for (i = 0; i < vi->numtxqs; i++) {
> +		struct virtqueue *svq = vi->sq[i]->svq;
> +
> +		while (1) {
> +			buf = virtqueue_detach_unused_buf(svq);
> +			if (!buf)
> +				break;
> +			dev_kfree_skb(buf);
> +		}
> +	}

I know this isn't your code, but it's ugly :)

        while ((buf = virtqueue_detach_unused_buf(svq)) != NULL)
                dev_kfree_skb(buf);

> +	for (i = 0; i < vi->numtxqs; i++) {
> +		struct virtqueue *rvq = vi->rq[i]->rvq;
> +
> +		while (1) {
> +			buf = virtqueue_detach_unused_buf(rvq);
> +			if (!buf)
> +				break;

Here too...

> +#define MAX_DEVICE_NAME		16

This isn't a good idea, see below.

> +static int initialize_vqs(struct virtnet_info *vi, int numtxqs)
> +{
> +	vq_callback_t **callbacks;
> +	struct virtqueue **vqs;
> +	int i, err = -ENOMEM;
> +	int totalvqs;
> +	char **names;

This whole routine is really messy.  How about doing find_vqs first,
then have routines like setup_rxq(), setup_txq() and setup_controlq()
would make this neater:

        static int setup_rxq(struct send_queue *sq, char *name);

Also, use kasprintf() instead of kmalloc & sprintf.

> +#if 1
> +	/* Allocate/initialize parameters for recv/send virtqueues */

Why is this #if 1'd?

I do prefer the #else method of doing two loops, myself (but use
kasprintf).

Cheers,
Rusty.
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