Re: [PATCH vhost v2 19/19] virtio_net: sq support premapped mode

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Feb 23, 2024 at 04:27:26PM +0800, Xuan Zhuo wrote:
> If the xsk is enabling, the xsk tx will share the send queue.
> But the xsk requires that the send queue use the premapped mode.
> So the send queue must support premapped mode.
> 
> cmd:
>     sh samples/pktgen/pktgen_sample01_simple.sh -i eth0 \
>         -s 16 -d 10.0.0.128 -m 00:16:3e:2c:c8:2e -n 0 -p 100
> CPU:
>     Intel(R) Xeon(R) Platinum 8369B CPU @ 2.70GHz
> 
> Machine:
>     ecs.g7.2xlarge(Aliyun)
> 
> before:              1600010.00
> after(no-premapped): 1599966.00
> after(premapped):    1600014.00
> 
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
>  drivers/net/virtio_net.c | 136 +++++++++++++++++++++++++++++++++++++--
>  1 file changed, 132 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 7715bb7032ec..b83ef6afc4fb 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -146,6 +146,25 @@ struct virtnet_rq_dma {
>  	u16 need_sync;
>  };
>  
> +struct virtnet_sq_dma {
> +	union {
> +		struct virtnet_sq_dma *next;
> +		void *data;
> +	};
> +
> +	u32 num;
> +
> +	dma_addr_t addr[MAX_SKB_FRAGS + 2];
> +	u32 len[MAX_SKB_FRAGS + 2];
> +};
> +
> +struct virtnet_sq_dma_head {
> +	/* record for kfree */
> +	void *p;
> +
> +	struct virtnet_sq_dma *free;
> +};
> +
>  /* Internal representation of a send virtqueue */
>  struct send_queue {
>  	/* Virtqueue associated with this send _queue */
> @@ -165,6 +184,8 @@ struct send_queue {
>  
>  	/* Record whether sq is in reset state. */
>  	bool reset;
> +
> +	struct virtnet_sq_dma_head dmainfo;
>  };
>  
>  /* Internal representation of a receive virtqueue */
> @@ -368,6 +389,95 @@ static struct xdp_frame *ptr_to_xdp(void *ptr)
>  	return (struct xdp_frame *)((unsigned long)ptr & ~VIRTIO_XDP_FLAG);
>  }
>  
> +static struct virtnet_sq_dma *virtnet_sq_unmap(struct send_queue *sq, void **data)
> +{
> +	struct virtnet_sq_dma *d;
> +	int i;
> +
> +	d = *data;
> +	*data = d->data;
> +
> +	for (i = 0; i < d->num; ++i)
> +		virtqueue_dma_unmap_page_attrs(sq->vq, d->addr[i], d->len[i],
> +					       DMA_TO_DEVICE, 0);
> +
> +	d->next = sq->dmainfo.free;
> +	sq->dmainfo.free = d;
> +
> +	return d;
> +}
> +
> +static struct virtnet_sq_dma *virtnet_sq_map_sg(struct send_queue *sq,
> +						int nents, void *data)
> +{
> +	struct virtnet_sq_dma *d;
> +	struct scatterlist *sg;
> +	int i;
> +
> +	if (!sq->dmainfo.free)
> +		return NULL;
> +
> +	d = sq->dmainfo.free;
> +	sq->dmainfo.free = d->next;
> +
> +	for_each_sg(sq->sg, sg, nents, i) {
> +		if (virtqueue_dma_map_sg_attrs(sq->vq, sg, DMA_TO_DEVICE, 0))
> +			goto err;
> +
> +		d->addr[i] = sg->dma_address;
> +		d->len[i] = sg->length;
> +	}
> +
> +	d->data = data;
> +	d->num = i;
> +	return d;
> +
> +err:
> +	d->num = i;
> +	virtnet_sq_unmap(sq, (void **)&d);
> +	return NULL;
> +}


Do I see a reimplementation of linux/llist.h here?


> +
> +static int virtnet_add_outbuf(struct send_queue *sq, u32 num, void *data)
> +{
> +	int ret;
> +
> +	if (sq->vq->premapped) {
> +		data = virtnet_sq_map_sg(sq, num, data);
> +		if (!data)
> +			return -ENOMEM;
> +	}
> +
> +	ret = virtqueue_add_outbuf(sq->vq, sq->sg, num, data, GFP_ATOMIC);
> +	if (ret && sq->vq->premapped)
> +		virtnet_sq_unmap(sq, &data);
> +
> +	return ret;
> +}
> +
> +static int virtnet_sq_init_dma_mate(struct send_queue *sq)

Mate? The popular south african drink?

> +{
> +	struct virtnet_sq_dma *d;
> +	int num, i;
> +
> +	num = virtqueue_get_vring_size(sq->vq);
> +
> +	sq->dmainfo.free = kcalloc(num, sizeof(*sq->dmainfo.free), GFP_KERNEL);
> +	if (!sq->dmainfo.free)
> +		return -ENOMEM;


This could be quite a bit of memory for a large queue.  And for a bunch
of common cases where unmap is a nop (e.g. iommu pt) this does nothing
useful at all.  And also, this does nothing useful if PLATFORM_ACCESS is off
which is super common.

A while ago I proposed:
- extend DMA APIs so one can query whether unmap is a nop
  and whether sync is a nop
- virtio wrapper taking into account PLATFORM_ACCESS too

then we can save all this work and memory when not needed.



> +
> +	sq->dmainfo.p = sq->dmainfo.free;
> +
> +	for (i = 0; i < num; ++i) {
> +		d = &sq->dmainfo.free[i];
> +		d->next = d + 1;
> +	}
> +
> +	d->next = NULL;
> +
> +	return 0;
> +}
> +
>  static void __free_old_xmit(struct send_queue *sq, bool in_napi,
>  			    struct virtnet_sq_free_stats *stats)
>  {
> @@ -377,6 +487,9 @@ static void __free_old_xmit(struct send_queue *sq, bool in_napi,
>  	while ((ptr = virtqueue_get_buf(sq->vq, &len)) != NULL) {
>  		++stats->packets;
>  
> +		if (sq->vq->premapped)
> +			virtnet_sq_unmap(sq, &ptr);
> +
>  		if (!is_xdp_frame(ptr)) {
>  			struct sk_buff *skb = ptr;
>  
> @@ -890,8 +1003,7 @@ static int __virtnet_xdp_xmit_one(struct virtnet_info *vi,
>  			    skb_frag_size(frag), skb_frag_off(frag));
>  	}
>  
> -	err = virtqueue_add_outbuf(sq->vq, sq->sg, nr_frags + 1,
> -				   xdp_to_ptr(xdpf), GFP_ATOMIC);
> +	err = virtnet_add_outbuf(sq, nr_frags + 1, xdp_to_ptr(xdpf));
>  	if (unlikely(err))
>  		return -ENOSPC; /* Caller handle free/refcnt */
>  
> @@ -2357,7 +2469,7 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
>  			return num_sg;
>  		num_sg++;
>  	}
> -	return virtqueue_add_outbuf(sq->vq, sq->sg, num_sg, skb, GFP_ATOMIC);
> +	return virtnet_add_outbuf(sq, num_sg, skb);
>  }
>  
>  static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> @@ -4166,6 +4278,8 @@ static void virtnet_free_queues(struct virtnet_info *vi)
>  	for (i = 0; i < vi->max_queue_pairs; i++) {
>  		__netif_napi_del(&vi->rq[i].napi);
>  		__netif_napi_del(&vi->sq[i].napi);
> +
> +		kfree(vi->sq[i].dmainfo.p);
>  	}
>  
>  	/* We called __netif_napi_del(),
> @@ -4214,6 +4328,15 @@ static void free_receive_page_frags(struct virtnet_info *vi)
>  
>  static void virtnet_sq_free_unused_buf(struct virtqueue *vq, void *buf)
>  {
> +	struct virtnet_info *vi = vq->vdev->priv;
> +	struct send_queue *sq;
> +	int i = vq2rxq(vq);
> +
> +	sq = &vi->sq[i];
> +
> +	if (sq->vq->premapped)
> +		virtnet_sq_unmap(sq, &buf);
> +
>  	if (!is_xdp_frame(buf))
>  		dev_kfree_skb(buf);
>  	else
> @@ -4327,8 +4450,10 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>  		if (ctx)
>  			ctx[rxq2vq(i)] = true;
>  
> -		if (premapped)
> +		if (premapped) {
>  			premapped[rxq2vq(i)] = true;
> +			premapped[txq2vq(i)] = true;
> +		}
>  	}
>  
>  	cfg.nvqs      = total_vqs;
> @@ -4352,6 +4477,9 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>  		vi->rq[i].vq = vqs[rxq2vq(i)];
>  		vi->rq[i].min_buf_len = mergeable_min_buf_len(vi, vi->rq[i].vq);
>  		vi->sq[i].vq = vqs[txq2vq(i)];
> +
> +		if (vi->sq[i].vq->premapped)
> +			virtnet_sq_init_dma_mate(&vi->sq[i]);
>  	}
>  
>  	/* run here: ret == 0. */
> -- 
> 2.32.0.3.g01195cf9f