Re: [PATCH net-next v2 06/13] virtio-net: rq submits premapped per-buffer

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

On Wed, 6 Nov 2024 09:56:55 +0800, Jason Wang <jasowang@redhat.com> wrote:
> On Tue, Nov 5, 2024 at 3:23 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >
> > On Tue, 5 Nov 2024 11:23:50 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > On Wed, Oct 30, 2024 at 4:25 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > > >
> > > > virtio-net rq submits premapped per-buffer by setting sg page to NULL;
> > > >
> > > > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > > > ---
> > > >  drivers/net/virtio_net.c | 24 +++++++++++++-----------
> > > >  1 file changed, 13 insertions(+), 11 deletions(-)
> > > >
> > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > > index 792e9eadbfc3..09757fa408bd 100644
> > > > --- a/drivers/net/virtio_net.c
> > > > +++ b/drivers/net/virtio_net.c
> > > > @@ -542,6 +542,12 @@ static struct sk_buff *ptr_to_skb(void *ptr)
> > > >         return (struct sk_buff *)((unsigned long)ptr & ~VIRTIO_ORPHAN_FLAG);
> > > >  }
> > > >
> > > > +static void sg_fill_dma(struct scatterlist *sg, dma_addr_t addr, u32 len)
> > > > +{
> > > > +       sg->dma_address = addr;
> > > > +       sg->length = len;
> > >
> > > This may work but I think it's better to reuse existing dma sg helpers like:
> > >
> > > sg_dma_address(sg) = addr;
> > > sg_dma_length(sg) = len;
> > >
> > > And we probably need to fix the virtio core which only uses
> > > sg_dma_address() but not sg_dma_length().
> > >
> > > This helps us to avoid future issues when CONFIG_NEED_SG_DMA_LENGTH is set.
> >
> >
> > I don't think so.
> >
> > For no-premapped mode, we pass the sg as no-dma sg to virtio core,
> > so the virtio core uses the sg->length directly.
>
> This is fine.
>
> > If virtio core do dma map for sg, we do not use the dma_mag_sg_attrs(),
> > so we must use sg->length directly.
>
> I meant it's a hack. It may work now but will be a bug in the future.
>
> For example, I'm playing a prototype to do pre mapping for virtio-blk,
> the idea is to move the expensive DMA mappings in the case of swiotlb
> etc to be done outside the pre virtqueue lock. In that case, the
> driver may want to use dma_map_sg() instead of dma_map_page().
>
> I'd suppose we will finally go with the way where DMA mappings needs
> to be handled by the driver, and dma_map_sg() is faster than per sg
> dma_map_page() anyhow.
>
> >
> > In this case, for the driver, we can not use sg_dma_length(),
> > if CONFIG_NEED_SG_DMA_LENGTH is set, sg_dma_length() will set sg->dma_length,
> > but virtio core use sg->length.
>
> Well, we just need a minor tweak to get the length from
> vring_map_one_sg(), then everything should be fine?
>
> if (sg_is_premapped) {
>       *addr = sg_dma_address(sg);
>       *len = sg_dma_len(sg);
> }

For now, let us start from:

 if (sg_is_premapped) {
       *addr = sg_dma_address(sg);
       sg->length = sg_dma_len(sg);
 }

Then virtio core needs to be refactor to use dma_map_sg() in future.

Thanks.


>
> >
> > For sg->dma_address, it is ok for me to use sg_dma_address or not.
> > But for consistency to sg->length, I use the sg->dma_address directly.
> >
> > I noticed this is special, so I put them into an independent function.
> >
> > Thanks.
>
> Actually, the code like sg_fill_dma() calls for a virtqueue dma
> mapping helper, I think we've agreed that core needs to hide DMA
> details from the driver.  That is something like
> virtqueue_dma_map_sg() etc.
>
> Thanks
>
> >
> > >
> > > Others look good.
> > >
> > > Thanks
> > >
> > > > +}
> > > > +
> > > >  static void __free_old_xmit(struct send_queue *sq, struct netdev_queue *txq,
> > > >                             bool in_napi, struct virtnet_sq_free_stats *stats)
> > > >  {
> > > > @@ -915,8 +921,7 @@ static void virtnet_rq_init_one_sg(struct receive_queue *rq, void *buf, u32 len)
> > > >         addr = dma->addr - sizeof(*dma) + offset;
> > > >
> > > >         sg_init_table(rq->sg, 1);
> > > > -       rq->sg[0].dma_address = addr;
> > > > -       rq->sg[0].length = len;
> > > > +       sg_fill_dma(rq->sg, addr, len);
> > > >  }
> > > >
> > > >  static void *virtnet_rq_alloc(struct receive_queue *rq, u32 size, gfp_t gfp)
> > > > @@ -1068,12 +1073,6 @@ static void check_sq_full_and_disable(struct virtnet_info *vi,
> > > >         }
> > > >  }
> > > >
> > > > -static void sg_fill_dma(struct scatterlist *sg, dma_addr_t addr, u32 len)
> > > > -{
> > > > -       sg->dma_address = addr;
> > > > -       sg->length = len;
> > > > -}
> > > > -
> > > >  static struct xdp_buff *buf_to_xdp(struct virtnet_info *vi,
> > > >                                    struct receive_queue *rq, void *buf, u32 len)
> > > >  {
> > > > @@ -1354,7 +1353,8 @@ static int virtnet_add_recvbuf_xsk(struct virtnet_info *vi, struct receive_queue
> > > >                 sg_init_table(rq->sg, 1);
> > > >                 sg_fill_dma(rq->sg, addr, len);
> > > >
> > > > -               err = virtqueue_add_inbuf(rq->vq, rq->sg, 1, xsk_buffs[i], gfp);
> > > > +               err = virtqueue_add_inbuf_premapped(rq->vq, rq->sg, 1, xsk_buffs[i],
> > > > +                                                   NULL, true, gfp);
> > > >                 if (err)
> > > >                         goto err;
> > > >         }
> > > > @@ -2431,7 +2431,8 @@ static int add_recvbuf_small(struct virtnet_info *vi, struct receive_queue *rq,
> > > >
> > > >         virtnet_rq_init_one_sg(rq, buf, vi->hdr_len + GOOD_PACKET_LEN);
> > > >
> > > > -       err = virtqueue_add_inbuf_ctx(rq->vq, rq->sg, 1, buf, ctx, gfp);
> > > > +       err = virtqueue_add_inbuf_premapped(rq->vq, rq->sg, 1, buf, ctx,
> > > > +                                           rq->do_dma, gfp);
> > > >         if (err < 0) {
> > > >                 if (rq->do_dma)
> > > >                         virtnet_rq_unmap(rq, buf, 0);
> > > > @@ -2546,7 +2547,8 @@ static int add_recvbuf_mergeable(struct virtnet_info *vi,
> > > >         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);
> > > > +       err = virtqueue_add_inbuf_premapped(rq->vq, rq->sg, 1, buf, ctx,
> > > > +                                           rq->do_dma, gfp);
> > > >         if (err < 0) {
> > > >                 if (rq->do_dma)
> > > >                         virtnet_rq_unmap(rq, buf, 0);
> > > > --
> > > > 2.32.0.3.g01195cf9f
> > > >
> > >
> >
>