Re: [PATCH net-next 07/13] virtio_net: refactor the xmit type

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

On Wed, 11 Sep 2024 12:04:16 +0800, Jason Wang <jasowang@redhat.com> wrote:
> On Tue, Aug 20, 2024 at 3:33 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >
> > Because the af-xdp will introduce a new xmit type, so I refactor the
> > xmit type mechanism first.
> >
> > We use the last two bits of the pointer to distinguish the xmit type,
> > so we can distinguish four xmit types. Now we have three types: skb,
> > orphan and xdp.
>
> And if I was not wrong, we do not anymore use bitmasks. If yes, let's
> explain the reason here.

In general, pointers are aligned to 4 or 8 bytes. If it is aligned to 4 bytes,
then only two bits are free for a pointer. So we can only use two bits.

But there are 4 types here, so we can't use bits to distinguish them.

b00 for skb
b01 for SKB_ORPHAN
b10 for XDP
b11 for af-xdp tx


>
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > ---
> >  drivers/net/virtio_net.c | 90 +++++++++++++++++++++++-----------------
> >  1 file changed, 51 insertions(+), 39 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 41aaea3b90fd..96abee36738b 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -45,9 +45,6 @@ module_param(napi_tx, bool, 0644);
> >  #define VIRTIO_XDP_TX          BIT(0)
> >  #define VIRTIO_XDP_REDIR       BIT(1)
> >
> > -#define VIRTIO_XDP_FLAG                BIT(0)
> > -#define VIRTIO_ORPHAN_FLAG     BIT(1)
> > -
> >  /* RX packet size EWMA. The average packet size is used to determine the packet
> >   * buffer size when refilling RX rings. As the entire RX ring may be refilled
> >   * at once, the weight is chosen so that the EWMA will be insensitive to short-
> > @@ -509,34 +506,35 @@ static struct sk_buff *virtnet_skb_append_frag(struct sk_buff *head_skb,
> >                                                struct page *page, void *buf,
> >                                                int len, int truesize);
> >
> > -static bool is_xdp_frame(void *ptr)
> > -{
> > -       return (unsigned long)ptr & VIRTIO_XDP_FLAG;
> > -}
> > +enum virtnet_xmit_type {
> > +       VIRTNET_XMIT_TYPE_SKB,
> > +       VIRTNET_XMIT_TYPE_ORPHAN,
>
> Let's rename this to SKB_ORPHAN?
>
> > +       VIRTNET_XMIT_TYPE_XDP,
> > +};
> >
> > -static void *xdp_to_ptr(struct xdp_frame *ptr)
> > -{
> > -       return (void *)((unsigned long)ptr | VIRTIO_XDP_FLAG);
> > -}
> > +#define VIRTNET_XMIT_TYPE_MASK (VIRTNET_XMIT_TYPE_SKB | VIRTNET_XMIT_TYPE_ORPHAN \
> > +                               | VIRTNET_XMIT_TYPE_XDP)
>

Maybe I should define VIRTNET_XMIT_TYPE_MASK to 0x3 directly with some comments.

Thanks.


> I may miss something but it seems not a correct bitmask definition as
> each member is not a bit actually?
>
> >
> > -static struct xdp_frame *ptr_to_xdp(void *ptr)
> > +static enum virtnet_xmit_type virtnet_xmit_ptr_strip(void **ptr)
> >  {
> > -       return (struct xdp_frame *)((unsigned long)ptr & ~VIRTIO_XDP_FLAG);
> > -}
> > +       unsigned long p = (unsigned long)*ptr;
> >
> > -static bool is_orphan_skb(void *ptr)
> > -{
> > -       return (unsigned long)ptr & VIRTIO_ORPHAN_FLAG;
> > +       *ptr = (void *)(p & ~VIRTNET_XMIT_TYPE_MASK);
> > +
> > +       return p & VIRTNET_XMIT_TYPE_MASK;
> >  }
> >
> > -static void *skb_to_ptr(struct sk_buff *skb, bool orphan)
> > +static void *virtnet_xmit_ptr_mix(void *ptr, enum virtnet_xmit_type type)
> >  {
> > -       return (void *)((unsigned long)skb | (orphan ? VIRTIO_ORPHAN_FLAG : 0));
> > +       return (void *)((unsigned long)ptr | type);
> >  }
> >
> > -static struct sk_buff *ptr_to_skb(void *ptr)
> > +static int virtnet_add_outbuf(struct send_queue *sq, int num, void *data,
> > +                             enum virtnet_xmit_type type)
> >  {
> > -       return (struct sk_buff *)((unsigned long)ptr & ~VIRTIO_ORPHAN_FLAG);
> > +       return virtqueue_add_outbuf(sq->vq, sq->sg, num,
> > +                                   virtnet_xmit_ptr_mix(data, type),
> > +                                   GFP_ATOMIC);
> >  }
> >
> >  static void sg_fill_dma(struct scatterlist *sg, dma_addr_t addr, u32 len)
> > @@ -549,29 +547,37 @@ static void sg_fill_dma(struct scatterlist *sg, dma_addr_t addr, u32 len)
> >  static void __free_old_xmit(struct send_queue *sq, struct netdev_queue *txq,
> >                             bool in_napi, struct virtnet_sq_free_stats *stats)
> >  {
> > +       struct xdp_frame *frame;
> > +       struct sk_buff *skb;
> >         unsigned int len;
> >         void *ptr;
> >
> >         while ((ptr = virtqueue_get_buf(sq->vq, &len)) != NULL) {
> > -               if (!is_xdp_frame(ptr)) {
> > -                       struct sk_buff *skb = ptr_to_skb(ptr);
> > +               switch (virtnet_xmit_ptr_strip(&ptr)) {
> > +               case VIRTNET_XMIT_TYPE_SKB:
> > +                       skb = ptr;
> >
> >                         pr_debug("Sent skb %p\n", skb);
> > +                       stats->napi_packets++;
> > +                       stats->napi_bytes += skb->len;
> > +                       napi_consume_skb(skb, in_napi);
> > +                       break;
> >
> > -                       if (is_orphan_skb(ptr)) {
> > -                               stats->packets++;
> > -                               stats->bytes += skb->len;
> > -                       } else {
> > -                               stats->napi_packets++;
> > -                               stats->napi_bytes += skb->len;
> > -                       }
> > +               case VIRTNET_XMIT_TYPE_ORPHAN:
> > +                       skb = ptr;
> > +
> > +                       stats->packets++;
> > +                       stats->bytes += skb->len;
> >                         napi_consume_skb(skb, in_napi);
> > -               } else {
> > -                       struct xdp_frame *frame = ptr_to_xdp(ptr);
> > +                       break;
> > +
> > +               case VIRTNET_XMIT_TYPE_XDP:
> > +                       frame = ptr;
> >
> >                         stats->packets++;
> >                         stats->bytes += xdp_get_frame_len(frame);
> >                         xdp_return_frame(frame);
> > +                       break;
> >                 }
> >         }
> >         netdev_tx_completed_queue(txq, stats->napi_packets, stats->napi_bytes);
> > @@ -1421,8 +1427,7 @@ 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, xdpf, VIRTNET_XMIT_TYPE_XDP);
> >         if (unlikely(err))
> >                 return -ENOSPC; /* Caller handle free/refcnt */
> >
> > @@ -3028,8 +3033,9 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb, bool orphan)
> >                         return num_sg;
> >                 num_sg++;
> >         }
> > -       return virtqueue_add_outbuf(sq->vq, sq->sg, num_sg,
> > -                                   skb_to_ptr(skb, orphan), GFP_ATOMIC);
> > +
> > +       return virtnet_add_outbuf(sq, num_sg, skb,
> > +                                 orphan ? VIRTNET_XMIT_TYPE_ORPHAN : VIRTNET_XMIT_TYPE_SKB);
> >  }
> >
> >  static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> > @@ -5906,10 +5912,16 @@ static void free_receive_page_frags(struct virtnet_info *vi)
> >
> >  static void virtnet_sq_free_unused_buf(struct virtqueue *vq, void *buf)
> >  {
> > -       if (!is_xdp_frame(buf))
> > +       switch (virtnet_xmit_ptr_strip(&buf)) {
> > +       case VIRTNET_XMIT_TYPE_SKB:
> > +       case VIRTNET_XMIT_TYPE_ORPHAN:
> >                 dev_kfree_skb(buf);
> > -       else
> > -               xdp_return_frame(ptr_to_xdp(buf));
> > +               break;
> > +
> > +       case VIRTNET_XMIT_TYPE_XDP:
> > +               xdp_return_frame(buf);
> > +               break;
> > +       }
> >  }
>
> Others look fine.
>
> Thanks
>
> >
> >  static void free_unused_bufs(struct virtnet_info *vi)
> > --
> > 2.32.0.3.g01195cf9f
> >
>