netdev - Re: [PATCH vhost v10 03/10] virtio_ring: split: support add premapped buf

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Date: Tue, 27 Jun 2023 17:01:31 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: Jason Wang <jasowang@...hat.com>
Cc: virtualization@...ts.linux-foundation.org,
 "Michael S. Tsirkin" <mst@...hat.com>,
 "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>,
 Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 John Fastabend <john.fastabend@...il.com>,
 netdev@...r.kernel.org,
 bpf@...r.kernel.org
Subject: Re: [PATCH vhost v10 03/10] virtio_ring: split: support add premapped buf

On Tue, 27 Jun 2023 16:03:26 +0800, Jason Wang <jasowang@...hat.com> wrote:
> On Fri, Jun 2, 2023 at 5:22 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> >
> > If the vq is the premapped mode, use the sg_dma_address() directly.
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > ---
> >  drivers/virtio/virtio_ring.c | 46 ++++++++++++++++++++++--------------
> >  1 file changed, 28 insertions(+), 18 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 2afdfb9e3e30..18212c3e056b 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -598,8 +598,12 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> >                 for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> >                         dma_addr_t addr;
> >
> > -                       if (vring_map_one_sg(vq, sg, DMA_TO_DEVICE, &addr))
> > -                               goto unmap_release;
> > +                       if (vq->premapped) {
> > +                               addr = sg_dma_address(sg);
> > +                       } else {
> > +                               if (vring_map_one_sg(vq, sg, DMA_TO_DEVICE, &addr))
> > +                                       goto unmap_release;
> > +                       }
>
> Btw, I wonder whether or not it would be simple to implement the
> vq->premapped check inside vring_map_one_sg() assuming the
> !use_dma_api is done there as well.


YES,

That will more simple for the caller.

But we will have things like:

int func(bool do)
{
if (!do)
    return;
}

I like this way, but you don't like it in last version.

>
> >
> >                         prev = i;
> >                         /* Note that we trust indirect descriptor
> > @@ -614,8 +618,12 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> >                 for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> >                         dma_addr_t addr;
> >
> > -                       if (vring_map_one_sg(vq, sg, DMA_FROM_DEVICE, &addr))
> > -                               goto unmap_release;
> > +                       if (vq->premapped) {
> > +                               addr = sg_dma_address(sg);
> > +                       } else {
> > +                               if (vring_map_one_sg(vq, sg, DMA_FROM_DEVICE, &addr))
> > +                                       goto unmap_release;
> > +                       }
> >
> >                         prev = i;
> >                         /* Note that we trust indirect descriptor
> > @@ -689,21 +697,23 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> >         return 0;
> >
> >  unmap_release:
> > -       err_idx = i;
> > +       if (!vq->premapped) {
>
> Can vq->premapped be true here? The label is named as "unmap_relase"
> which implies "map" beforehand which seems not the case for
> premapping.

I see.

Rethink about this, there is a better way.
I will fix in next version.


Thanks.


>
> Thanks
>
>
> > +               err_idx = i;
> >
> > -       if (indirect)
> > -               i = 0;
> > -       else
> > -               i = head;
> > -
> > -       for (n = 0; n < total_sg; n++) {
> > -               if (i == err_idx)
> > -                       break;
> > -               if (indirect) {
> > -                       vring_unmap_one_split_indirect(vq, &desc[i]);
> > -                       i = virtio16_to_cpu(_vq->vdev, desc[i].next);
> > -               } else
> > -                       i = vring_unmap_one_split(vq, i);
> > +               if (indirect)
> > +                       i = 0;
> > +               else
> > +                       i = head;
> > +
> > +               for (n = 0; n < total_sg; n++) {
> > +                       if (i == err_idx)
> > +                               break;
> > +                       if (indirect) {
> > +                               vring_unmap_one_split_indirect(vq, &desc[i]);
> > +                               i = virtio16_to_cpu(_vq->vdev, desc[i].next);
> > +                       } else
> > +                               i = vring_unmap_one_split(vq, i);
> > +               }
> >         }
> >
> >         if (indirect)
> > --
> > 2.32.0.3.g01195cf9f
> >
>