netdev - Re: [PATCH vhost v13 05/12] virtio_ring: introduce virtqueue_dma

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Message-ID: <1692003413.6339955-1-xuanzhuo@linux.alibaba.com>
Date: Mon, 14 Aug 2023 16:56:53 +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,
 Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH vhost v13 05/12] virtio_ring: introduce virtqueue_dma_dev()

On Mon, 14 Aug 2023 11:05:49 +0800, Jason Wang <jasowang@...hat.com> wrote:
> On Thu, Aug 10, 2023 at 8:31 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> >
> > Added virtqueue_dma_dev() to get DMA device for virtio. Then the
> > caller can do dma operation in advance. The purpose is to keep memory
> > mapped across multiple add/get buf operations.
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > Acked-by: Jason Wang <jasowang@...hat.com>
>
> So I think we don't have actual users for this in this series? Can we
> simply have another independent patch for this?

I am ok. I will remove this from the next version.

But I also help merge this to 6.6. Then we can let the virtio-net to support
AF_XDP in 6.7+.

>
> > ---
> >  drivers/virtio/virtio_ring.c | 17 +++++++++++++++++
> >  include/linux/virtio.h       |  2 ++
> >  2 files changed, 19 insertions(+)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index f9f772e85a38..bb3d73d221cd 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -2265,6 +2265,23 @@ int virtqueue_add_inbuf_ctx(struct virtqueue *vq,
> >  }
> >  EXPORT_SYMBOL_GPL(virtqueue_add_inbuf_ctx);
> >
> > +/**
> > + * virtqueue_dma_dev - get the dma dev
> > + * @_vq: the struct virtqueue we're talking about.
> > + *
> > + * Returns the dma dev. That can been used for dma api.
> > + */
> > +struct device *virtqueue_dma_dev(struct virtqueue *_vq)
> > +{
> > +       struct vring_virtqueue *vq = to_vvq(_vq);
> > +
> > +       if (vq->use_dma_api)
> > +               return vring_dma_dev(vq);
> > +       else
> > +               return NULL;
> > +}
> > +EXPORT_SYMBOL_GPL(virtqueue_dma_dev);
>
> One possible concern is that exporting things like NULL may result in
> the switch in the caller (driver). I wonder if it's better to do
> BUG_ON() in the path of NULL?

I agree.

But we need a new helper to tell the driver(or AF_XDP) that the device support
ACCESS_PLATFORM or not.

We need a switch, but we can make the switch is irrelevant to the DMA.

Thanks.

>
> Thanks
>
> > +
> >  /**
> >   * virtqueue_kick_prepare - first half of split virtqueue_kick call.
> >   * @_vq: the struct virtqueue
> > diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> > index 8add38038877..bd55a05eec04 100644
> > --- a/include/linux/virtio.h
> > +++ b/include/linux/virtio.h
> > @@ -61,6 +61,8 @@ int virtqueue_add_sgs(struct virtqueue *vq,
> >                       void *data,
> >                       gfp_t gfp);
> >
> > +struct device *virtqueue_dma_dev(struct virtqueue *vq);
> > +
> >  bool virtqueue_kick(struct virtqueue *vq);
> >
> >  bool virtqueue_kick_prepare(struct virtqueue *vq);
> > --
> > 2.32.0.3.g01195cf9f
> >
>