Re: [PATCH net-next v5 08/15] virtio_net: sq support premapped mode

[Jason Wang <jasowang@redhat.com>]

On Mon, Jun 17, 2024 at 3:39 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>
> On Mon, 17 Jun 2024 13:00:13 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > On Fri, Jun 14, 2024 at 2:39 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> 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 when it is bound to
> > > af-xdp.
> > >
> > > * virtnet_sq_set_premapped(sq, true) is used to enable premapped mode.
> > >
> > >     In this mode, the driver will record the dma info when skb or xdp
> > >     frame is sent.
> > >
> > >     Currently, the SQ premapped mode is operational only with af-xdp. In
> > >     this mode, af-xdp, the kernel stack, and xdp tx/redirect will share
> > >     the same SQ. Af-xdp independently manages its DMA. The kernel stack
> > >     and xdp tx/redirect utilize this DMA metadata to manage the DMA
> > >     info.
> > >
> > >     If the indirect descriptor feature be supported, the volume of DMA
> > >     details we need to maintain becomes quite substantial. Here, we have
> > >     a cap on the amount of DMA info we manage.
> > >
> > >     If the kernel stack and xdp tx/redirect attempt to use more
> > >     descriptors, virtnet_add_outbuf() will return an -ENOMEM error. But
> > >     the af-xdp can work continually.
> >
> > Rethink of this whole logic, it looks like all the complication came
> > as we decided to go with a pre queue pre mapping flag. I wonder if
> > things could be simplified if we do that per buffer?
>
> YES. That will be simply.
>
> Then this patch will be not needed. The virtio core must record the premapped
> imfo to the virtio ring state or extra.
>
>          http://lore.kernel.org/all/20230517022249.20790-6-xuanzhuo@linux.alibaba.com

Yes, something like this. I think it's worthwhile to re-consider that
approach. If my memory is correct, we haven't spotted the complicated
issues we need to deal with like this patch.

>
> >
> > Then we don't need complex logic like dmainfo and cap.
>
> So the premapped mode and the internal dma mode can coexist.
> Then we do not need to make the sq to support the premapped mode.

Probably.

>
>
> >
> > >
> > > * virtnet_sq_set_premapped(sq, false) is used to disable premapped mode.
> > >
> > > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > > ---
> > >  drivers/net/virtio_net.c | 228 ++++++++++++++++++++++++++++++++++++++-
> > >  1 file changed, 224 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index e84a4624549b..88ab9ea1646f 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -25,6 +25,7 @@
> > >  #include <net/net_failover.h>
> > >  #include <net/netdev_rx_queue.h>
> > >  #include <net/netdev_queues.h>
> > > +#include <uapi/linux/virtio_ring.h>
> >
> > Why do we need this?
>
> for using VIRTIO_RING_F_INDIRECT_DESC

Ok. It's probably a hint that something like layer violation happens.
A specific driver should not know details about the ring layout ...

>
>
> >
> > >
> > >  static int napi_weight = NAPI_POLL_WEIGHT;
> > >  module_param(napi_weight, int, 0444);
> > > @@ -276,6 +277,26 @@ struct virtnet_rq_dma {
> > >         u16 need_sync;
> > >  };
> > >
> > > +struct virtnet_sq_dma {
> > > +       union {
> > > +               struct llist_node node;
> > > +               struct llist_head head;
> >
> > If we want to cap the #dmas, could we simply use an array instead of
> > the list here?
> >
> > > +               void *data;
> > > +       };
> > > +       dma_addr_t addr;
> > > +       u32 len;
> > > +       u8 num;
> > > +};
> > > +
> > > +struct virtnet_sq_dma_info {
> > > +       /* record for kfree */
> > > +       void *p;
> > > +
> > > +       u32 free_num;
> > > +
> > > +       struct llist_head free;
> > > +};
> > > +
> > >  /* Internal representation of a send virtqueue */
> > >  struct send_queue {
> > >         /* Virtqueue associated with this send _queue */
> > > @@ -295,6 +316,11 @@ struct send_queue {
> > >
> > >         /* Record whether sq is in reset state. */
> > >         bool reset;
> > > +
> > > +       /* SQ is premapped mode or not. */
> > > +       bool premapped;
> > > +
> > > +       struct virtnet_sq_dma_info dmainfo;
> > >  };
> > >
> > >  /* Internal representation of a receive virtqueue */
> > > @@ -492,9 +518,11 @@ static void virtnet_sq_free_unused_buf(struct virtqueue *vq, void *buf);
> > >  enum virtnet_xmit_type {
> > >         VIRTNET_XMIT_TYPE_SKB,
> > >         VIRTNET_XMIT_TYPE_XDP,
> > > +       VIRTNET_XMIT_TYPE_DMA,
> >
> > I think the name is confusing, how about TYPE_PREMAPPED?
> >
> > >  };
> > >
> > > -#define VIRTNET_XMIT_TYPE_MASK (VIRTNET_XMIT_TYPE_SKB | VIRTNET_XMIT_TYPE_XDP)
> > > +#define VIRTNET_XMIT_TYPE_MASK (VIRTNET_XMIT_TYPE_SKB | VIRTNET_XMIT_TYPE_XDP \
> > > +                               | VIRTNET_XMIT_TYPE_DMA)
> > >
> > >  static enum virtnet_xmit_type virtnet_xmit_ptr_strip(void **ptr)
> > >  {
> > > @@ -510,12 +538,180 @@ static void *virtnet_xmit_ptr_mix(void *ptr, enum virtnet_xmit_type type)
> > >         return (void *)((unsigned long)ptr | type);
> > >  }
> > >
> > > +static void virtnet_sq_unmap(struct send_queue *sq, void **data)
> > > +{
> > > +       struct virtnet_sq_dma *head, *tail, *p;
> > > +       int i;
> > > +
> > > +       head = *data;
> > > +
> > > +       p = head;
> > > +
> > > +       for (i = 0; i < head->num; ++i) {
> > > +               virtqueue_dma_unmap_page_attrs(sq->vq, p->addr, p->len,
> > > +                                              DMA_TO_DEVICE, 0);
> > > +               tail = p;
> > > +               p = llist_entry(llist_next(&p->node), struct virtnet_sq_dma, node);
> > > +       }
> > > +
> > > +       *data = tail->data;
> > > +
> > > +       __llist_add_batch(&head->node, &tail->node,  &sq->dmainfo.free);
> > > +
> > > +       sq->dmainfo.free_num += head->num;
> > > +}
> > > +
> > > +static void *virtnet_dma_chain_update(struct send_queue *sq,
> > > +                                     struct virtnet_sq_dma *head,
> > > +                                     struct virtnet_sq_dma *tail,
> > > +                                     u8 num, void *data)
> > > +{
> > > +       sq->dmainfo.free_num -= num;
> > > +       head->num = num;
> > > +
> > > +       tail->data = data;
> > > +
> > > +       return virtnet_xmit_ptr_mix(head, VIRTNET_XMIT_TYPE_DMA);
> > > +}
> > > +
> > > +static struct virtnet_sq_dma *virtnet_sq_map_sg(struct send_queue *sq, int num, void *data)
> > > +{
> > > +       struct virtnet_sq_dma *head = NULL, *p = NULL;
> > > +       struct scatterlist *sg;
> > > +       dma_addr_t addr;
> > > +       int i, err;
> > > +
> > > +       if (num > sq->dmainfo.free_num)
> > > +               return NULL;
> > > +
> > > +       for (i = 0; i < num; ++i) {
> > > +               sg = &sq->sg[i];
> > > +
> > > +               addr = virtqueue_dma_map_page_attrs(sq->vq, sg_page(sg),
> > > +                                                   sg->offset,
> > > +                                                   sg->length, DMA_TO_DEVICE,
> > > +                                                   0);
> > > +               err = virtqueue_dma_mapping_error(sq->vq, addr);
> > > +               if (err)
> > > +                       goto err;
> > > +
> > > +               sg->dma_address = addr;
> > > +
> > > +               p = llist_entry(llist_del_first(&sq->dmainfo.free),
> > > +                               struct virtnet_sq_dma, node);
> > > +
> > > +               p->addr = sg->dma_address;
> > > +               p->len = sg->length;
> >
> > I may miss something, but I don't see how we cap the total number of dmainfos.
>
> static void *virtnet_dma_chain_update(struct send_queue *sq,
>                                      struct virtnet_sq_dma *head,
>                                      struct virtnet_sq_dma *tail,
>                                      u8 num, void *data)
> {
>        sq->dmainfo.free_num -= num;
> ->       head->num = num;
>
>        tail->data = data;
>
>        return virtnet_xmit_ptr_mix(head, VIRTNET_XMIT_TYPE_DMA);
> }

Ok, speak too fast I guess it should be more like:

 if (num > sq->dmainfo.free_num)
               return NULL;

Thanks

>
>
>
> Thanks.
>
> >
> > Thanks
> >
>