Subject: Re: [PATCH vhost v13 12/12] virtio_net: merge dma operations when filling mergeable buffers

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 26 Sep 2023 12:01:34 -0400

On Thu, Aug 10, 2023 at 08:30:57PM +0800, Xuan Zhuo wrote:
> Currently, the virtio core will perform a dma operation for each
> buffer. Although, the same page may be operated multiple times.
> 
> This patch, the driver does the dma operation and manages the dma
> address based the feature premapped of virtio core.
> 
> This way, we can perform only one dma operation for the pages of the
> alloc frag. This is beneficial for the iommu device.
> 
> kernel command line: intel_iommu=on iommu.passthrough=0
> 
>        |  strict=0  | strict=1
> Before |  775496pps | 428614pps
> After  | 1109316pps | 742853pps
> 
> Signed-off-by: Xuan Zhuo <[email-masked]>

Hi Xuan Zhuo,
looks like this patch is causing regressions. Do you have time to debug
or should I revert?

> ---
>  drivers/net/virtio_net.c | 228 ++++++++++++++++++++++++++++++++++-----
>  1 file changed, 202 insertions(+), 26 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 486b5849033d..16adb5ef18f8 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -126,6 +126,14 @@ static const struct virtnet_stat_desc virtnet_rq_stats_desc[] = {
>  #define VIRTNET_SQ_STATS_LEN	ARRAY_SIZE(virtnet_sq_stats_desc)
>  #define VIRTNET_RQ_STATS_LEN	ARRAY_SIZE(virtnet_rq_stats_desc)
>  
> +/* The dma information of pages allocated at a time. */
> +struct virtnet_rq_dma {
> +	dma_addr_t addr;
> +	u32 ref;
> +	u16 len;
> +	u16 need_sync;
> +};
> +
>  /* Internal representation of a send virtqueue */
>  struct send_queue {
>  	/* Virtqueue associated with this send _queue */
> @@ -175,6 +183,12 @@ struct receive_queue {
>  	char name[16];
>  
>  	struct xdp_rxq_info xdp_rxq;
> +
> +	/* Record the last dma info to free after new pages is allocated. */
> +	struct virtnet_rq_dma *last_dma;
> +
> +	/* Do dma by self */
> +	bool do_dma;
>  };
>  
>  /* This structure can contain rss message with maximum settings for indirection table and keysize
> @@ -549,6 +563,156 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  	return skb;
>  }
>  
> +static void virtnet_rq_unmap(struct receive_queue *rq, void *buf, u32 len)
> +{
> +	struct page *page = virt_to_head_page(buf);
> +	struct virtnet_rq_dma *dma;
> +	void *head;
> +	int offset;
> +
> +	head = page_address(page);
> +
> +	dma = head;
> +
> +	--dma->ref;
> +
> +	if (dma->ref) {
> +		if (dma->need_sync && len) {
> +			offset = buf - (head + sizeof(*dma));
> +
> +			virtqueue_dma_sync_single_range_for_cpu(rq->vq, dma->addr, offset,
> +								len, DMA_FROM_DEVICE);
> +		}
> +
> +		return;
> +	}
> +
> +	virtqueue_dma_unmap_single_attrs(rq->vq, dma->addr, dma->len,
> +					 DMA_FROM_DEVICE, DMA_ATTR_SKIP_CPU_SYNC);
> +	put_page(page);
> +}
> +
> +static void *virtnet_rq_get_buf(struct receive_queue *rq, u32 *len, void **ctx)
> +{
> +	void *buf;
> +
> +	buf = virtqueue_get_buf_ctx(rq->vq, len, ctx);
> +	if (buf && rq->do_dma)
> +		virtnet_rq_unmap(rq, buf, *len);
> +
> +	return buf;
> +}
> +
> +static void *virtnet_rq_detach_unused_buf(struct receive_queue *rq)
> +{
> +	void *buf;
> +
> +	buf = virtqueue_detach_unused_buf(rq->vq);
> +	if (buf && rq->do_dma)
> +		virtnet_rq_unmap(rq, buf, 0);
> +
> +	return buf;
> +}
> +
> +static void virtnet_rq_init_one_sg(struct receive_queue *rq, void *buf, u32 len)
> +{
> +	struct virtnet_rq_dma *dma;
> +	dma_addr_t addr;
> +	u32 offset;
> +	void *head;
> +
> +	if (!rq->do_dma) {
> +		sg_init_one(rq->sg, buf, len);
> +		return;
> +	}
> +
> +	head = page_address(rq->alloc_frag.page);
> +
> +	offset = buf - head;
> +
> +	dma = head;
> +
> +	addr = dma->addr - sizeof(*dma) + offset;
> +
> +	sg_init_table(rq->sg, 1);
> +	rq->sg[0].dma_address = addr;
> +	rq->sg[0].length = len;
> +}
> +
> +static void *virtnet_rq_alloc(struct receive_queue *rq, u32 size, gfp_t gfp)
> +{
> +	struct page_frag *alloc_frag = &rq->alloc_frag;
> +	struct virtnet_rq_dma *dma;
> +	void *buf, *head;
> +	dma_addr_t addr;
> +
> +	if (unlikely(!skb_page_frag_refill(size, alloc_frag, gfp)))
> +		return NULL;
> +
> +	head = page_address(alloc_frag->page);
> +
> +	if (rq->do_dma) {
> +		dma = head;
> +
> +		/* new pages */
> +		if (!alloc_frag->offset) {
> +			if (rq->last_dma) {
> +				/* Now, the new page is allocated, the last dma
> +				 * will not be used. So the dma can be unmapped
> +				 * if the ref is 0.
> +				 */
> +				virtnet_rq_unmap(rq, rq->last_dma, 0);
> +				rq->last_dma = NULL;
> +			}
> +
> +			dma->len = alloc_frag->size - sizeof(*dma);
> +
> +			addr = virtqueue_dma_map_single_attrs(rq->vq, dma + 1,
> +							      dma->len, DMA_FROM_DEVICE, 0);
> +			if (virtqueue_dma_mapping_error(rq->vq, addr))
> +				return NULL;
> +
> +			dma->addr = addr;
> +			dma->need_sync = virtqueue_dma_need_sync(rq->vq, addr);
> +
> +			/* Add a reference to dma to prevent the entire dma from
> +			 * being released during error handling. This reference
> +			 * will be freed after the pages are no longer used.
> +			 */
> +			get_page(alloc_frag->page);
> +			dma->ref = 1;
> +			alloc_frag->offset = sizeof(*dma);
> +
> +			rq->last_dma = dma;
> +		}
> +
> +		++dma->ref;
> +	}
> +
> +	buf = head + alloc_frag->offset;
> +
> +	get_page(alloc_frag->page);
> +	alloc_frag->offset += size;
> +
> +	return buf;
> +}
> +
> +static void virtnet_rq_set_premapped(struct virtnet_info *vi)
> +{
> +	int i;
> +
> +	/* disable for big mode */
> +	if (!vi->mergeable_rx_bufs && vi->big_packets)
> +		return;
> +
> +	for (i = 0; i < vi->max_queue_pairs; i++) {
> +		if (virtqueue_set_dma_premapped(vi->rq[i].vq))
> +			continue;
> +
> +		vi->rq[i].do_dma = true;
> +	}
> +}
> +
>  static void free_old_xmit_skbs(struct send_queue *sq, bool in_napi)
>  {
>  	unsigned int len;
> @@ -835,7 +999,7 @@ static struct page *xdp_linearize_page(struct receive_queue *rq,
>  		void *buf;
>  		int off;
>  
> -		buf = virtqueue_get_buf(rq->vq, &buflen);
> +		buf = virtnet_rq_get_buf(rq, &buflen, NULL);
>  		if (unlikely(!buf))
>  			goto err_buf;
>  
> @@ -1126,7 +1290,7 @@ static int virtnet_build_xdp_buff_mrg(struct net_device *dev,
>  		return -EINVAL;
>  
>  	while (--*num_buf > 0) {
> -		buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx);
> +		buf = virtnet_rq_get_buf(rq, &len, &ctx);
>  		if (unlikely(!buf)) {
>  			pr_debug("%s: rx error: %d buffers out of %d missing\n",
>  				 dev->name, *num_buf,
> @@ -1351,7 +1515,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>  	while (--num_buf) {
>  		int num_skb_frags;
>  
> -		buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx);
> +		buf = virtnet_rq_get_buf(rq, &len, &ctx);
>  		if (unlikely(!buf)) {
>  			pr_debug("%s: rx error: %d buffers out of %d missing\n",
>  				 dev->name, num_buf,
> @@ -1414,7 +1578,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>  err_skb:
>  	put_page(page);
>  	while (num_buf-- > 1) {
> -		buf = virtqueue_get_buf(rq->vq, &len);
> +		buf = virtnet_rq_get_buf(rq, &len, NULL);
>  		if (unlikely(!buf)) {
>  			pr_debug("%s: rx error: %d buffers missing\n",
>  				 dev->name, num_buf);
> @@ -1524,7 +1688,6 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
>  static int add_recvbuf_small(struct virtnet_info *vi, struct receive_queue *rq,
>  			     gfp_t gfp)
>  {
> -	struct page_frag *alloc_frag = &rq->alloc_frag;
>  	char *buf;
>  	unsigned int xdp_headroom = virtnet_get_headroom(vi);
>  	void *ctx = (void *)(unsigned long)xdp_headroom;
> @@ -1533,17 +1696,21 @@ static int add_recvbuf_small(struct virtnet_info *vi, struct receive_queue *rq,
>  
>  	len = SKB_DATA_ALIGN(len) +
>  	      SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> -	if (unlikely(!skb_page_frag_refill(len, alloc_frag, gfp)))
> +
> +	buf = virtnet_rq_alloc(rq, len, gfp);
> +	if (unlikely(!buf))
>  		return -ENOMEM;
>  
> -	buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset;
> -	get_page(alloc_frag->page);
> -	alloc_frag->offset += len;
> -	sg_init_one(rq->sg, buf + VIRTNET_RX_PAD + xdp_headroom,
> -		    vi->hdr_len + GOOD_PACKET_LEN);
> +	virtnet_rq_init_one_sg(rq, buf + VIRTNET_RX_PAD + xdp_headroom,
> +			       vi->hdr_len + GOOD_PACKET_LEN);
> +
>  	err = virtqueue_add_inbuf_ctx(rq->vq, rq->sg, 1, buf, ctx, gfp);
> -	if (err < 0)
> +	if (err < 0) {
> +		if (rq->do_dma)
> +			virtnet_rq_unmap(rq, buf, 0);
>  		put_page(virt_to_head_page(buf));
> +	}
> +
>  	return err;
>  }
>  
> @@ -1620,23 +1787,22 @@ static int add_recvbuf_mergeable(struct virtnet_info *vi,
>  	unsigned int headroom = virtnet_get_headroom(vi);
>  	unsigned int tailroom = headroom ? sizeof(struct skb_shared_info) : 0;
>  	unsigned int room = SKB_DATA_ALIGN(headroom + tailroom);
> -	char *buf;
> +	unsigned int len, hole;
>  	void *ctx;
> +	char *buf;
>  	int err;
> -	unsigned int len, hole;
>  
>  	/* Extra tailroom is needed to satisfy XDP's assumption. This
>  	 * means rx frags coalescing won't work, but consider we've
>  	 * disabled GSO for XDP, it won't be a big issue.
>  	 */
>  	len = get_mergeable_buf_len(rq, &rq->mrg_avg_pkt_len, room);
> -	if (unlikely(!skb_page_frag_refill(len + room, alloc_frag, gfp)))
> +
> +	buf = virtnet_rq_alloc(rq, len + room, gfp);
> +	if (unlikely(!buf))
>  		return -ENOMEM;
>  
> -	buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset;
>  	buf += headroom; /* advance address leaving hole at front of pkt */
> -	get_page(alloc_frag->page);
> -	alloc_frag->offset += len + room;
>  	hole = alloc_frag->size - alloc_frag->offset;
>  	if (hole < len + room) {
>  		/* To avoid internal fragmentation, if there is very likely not
> @@ -1650,11 +1816,15 @@ static int add_recvbuf_mergeable(struct virtnet_info *vi,
>  		alloc_frag->offset += hole;
>  	}
>  
> -	sg_init_one(rq->sg, buf, len);
> +	virtnet_rq_init_one_sg(rq, buf, len);
> +
>  	ctx = mergeable_len_to_ctx(len + room, headroom);
>  	err = virtqueue_add_inbuf_ctx(rq->vq, rq->sg, 1, buf, ctx, gfp);
> -	if (err < 0)
> +	if (err < 0) {
> +		if (rq->do_dma)
> +			virtnet_rq_unmap(rq, buf, 0);
>  		put_page(virt_to_head_page(buf));
> +	}
>  
>  	return err;
>  }
> @@ -1775,13 +1945,13 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
>  		void *ctx;
>  
>  		while (stats.packets < budget &&
> -		       (buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx))) {
> +		       (buf = virtnet_rq_get_buf(rq, &len, &ctx))) {
>  			receive_buf(vi, rq, buf, len, ctx, xdp_xmit, &stats);
>  			stats.packets++;
>  		}
>  	} else {
>  		while (stats.packets < budget &&
> -		       (buf = virtqueue_get_buf(rq->vq, &len)) != NULL) {
> +		       (buf = virtnet_rq_get_buf(rq, &len, NULL)) != NULL) {
>  			receive_buf(vi, rq, buf, len, NULL, xdp_xmit, &stats);
>  			stats.packets++;
>  		}
> @@ -3553,8 +3723,11 @@ static void free_receive_page_frags(struct virtnet_info *vi)
>  {
>  	int i;
>  	for (i = 0; i < vi->max_queue_pairs; i++)
> -		if (vi->rq[i].alloc_frag.page)
> +		if (vi->rq[i].alloc_frag.page) {
> +			if (vi->rq[i].do_dma && vi->rq[i].last_dma)
> +				virtnet_rq_unmap(&vi->rq[i], vi->rq[i].last_dma, 0);
>  			put_page(vi->rq[i].alloc_frag.page);
> +		}
>  }
>  
>  static void virtnet_sq_free_unused_buf(struct virtqueue *vq, void *buf)
> @@ -3591,9 +3764,10 @@ static void free_unused_bufs(struct virtnet_info *vi)
>  	}
>  
>  	for (i = 0; i < vi->max_queue_pairs; i++) {
> -		struct virtqueue *vq = vi->rq[i].vq;
> -		while ((buf = virtqueue_detach_unused_buf(vq)) != NULL)
> -			virtnet_rq_free_unused_buf(vq, buf);
> +		struct receive_queue *rq = &vi->rq[i];
> +
> +		while ((buf = virtnet_rq_detach_unused_buf(rq)) != NULL)
> +			virtnet_rq_free_unused_buf(rq->vq, buf);
>  		cond_resched();
>  	}
>  }
> @@ -3767,6 +3941,8 @@ static int init_vqs(struct virtnet_info *vi)
>  	if (ret)
>  		goto err_free;
>  
> +	virtnet_rq_set_premapped(vi);
> +
>  	cpus_read_lock();
>  	virtnet_set_affinity(vi);
>  	cpus_read_unlock();
> -- 
> 2.32.0.3.g01195cf9f