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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 26 Feb 2024 06:31:49 -0500

On Mon, Feb 26, 2024 at 07:20:40PM +0800, Xuan Zhuo wrote:
> On Sun, 25 Feb 2024 03:38:48 -0500, "Michael S. Tsirkin" <[email-masked]> wrote:
> > 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 <[email-masked]>
> > > ---
> > >  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
> 
> I think such code is ok:
> 
> bool dma_is_direct(struct device *dev)
> {
> 	if (!dma_map_direct(dev, ops))
> 		return false;

what is dma_map_direct? can't find it in the tree.

> 	if (is_swiotlb_force_bounce(dev))
> 		return false;
> 
> 	return true;
> }
> 
> @Christoph Hellwig
> 
> Thanks.
> 
> 
> >   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
> >