netdev - Re: [PATCH vhost v11 03/10] virtio_ring: introduce virtqueue_set

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Message-ID: <1689150956.767141-4-xuanzhuo@linux.alibaba.com>
Date: Wed, 12 Jul 2023 16:35:56 +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 v11 03/10] virtio_ring: introduce virtqueue_set_premapped()

On Wed, 12 Jul 2023 16:24:18 +0800, Jason Wang <jasowang@...hat.com> wrote:
> On Mon, Jul 10, 2023 at 11:42 AM Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> wrote:
>
> > This helper allows the driver change the dma mode to premapped mode.
> > Under the premapped mode, the virtio core do not do dma mapping
> > internally.
> >
> > This just work when the use_dma_api is true. If the use_dma_api is false,
> > the dma options is not through the DMA APIs, that is not the standard
> > way of the linux kernel.
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > ---
> >  drivers/virtio/virtio_ring.c | 45 ++++++++++++++++++++++++++++++++++++
> >  include/linux/virtio.h       |  2 ++
> >  2 files changed, 47 insertions(+)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 87d7ceeecdbd..5ace4539344c 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -172,6 +172,9 @@ struct vring_virtqueue {
> >         /* Host publishes avail event idx */
> >         bool event;
> >
> > +       /* Do DMA mapping by driver */
> > +       bool premapped;
> > +
> >         /* Head of free buffer list. */
> >         unsigned int free_head;
> >         /* Number we've added since last sync. */
> > @@ -2061,6 +2064,7 @@ static struct virtqueue
> > *vring_create_virtqueue_packed(
> >         vq->packed_ring = true;
> >         vq->dma_dev = dma_dev;
> >         vq->use_dma_api = vring_use_dma_api(vdev);
> > +       vq->premapped = false;
> >
> >         vq->indirect = virtio_has_feature(vdev,
> > VIRTIO_RING_F_INDIRECT_DESC) &&
> >                 !context;
> > @@ -2550,6 +2554,7 @@ static struct virtqueue
> > *__vring_new_virtqueue(unsigned int index,
> >  #endif
> >         vq->dma_dev = dma_dev;
> >         vq->use_dma_api = vring_use_dma_api(vdev);
> > +       vq->premapped = false;
> >
> >         vq->indirect = virtio_has_feature(vdev,
> > VIRTIO_RING_F_INDIRECT_DESC) &&
> >                 !context;
> > @@ -2693,6 +2698,46 @@ int virtqueue_resize(struct virtqueue *_vq, u32 num,
> >  }
> >  EXPORT_SYMBOL_GPL(virtqueue_resize);
> >
> > +/**
> > + * virtqueue_set_premapped - set the vring premapped mode
> > + * @_vq: the struct virtqueue we're talking about.
> > + *
> > + * Enable the premapped mode of the vq.
> > + *
> > + * The vring in premapped mode does not do dma internally, so the driver
> > must
> > + * do dma mapping in advance. The driver must pass the dma_address through
> > + * dma_address of scatterlist. When the driver got a used buffer from
> > + * the vring, it has to unmap the dma address.
> > + *
> > + * This function must be called immediately after creating the vq, or
> > after vq
> > + * reset, and before adding any buffers to it.
> > + *
> > + * Caller must ensure we don't call this with other virtqueue operations
> > + * at the same time (except where noted).
> > + *
> > + * Returns zero or a negative error.
> > + * 0: success.
> > + * -EINVAL: vring does not use the dma api, so we can not enable
> > premapped mode.
> > + */
> > +int virtqueue_set_premapped(struct virtqueue *_vq)
> > +{
> > +       struct vring_virtqueue *vq = to_vvq(_vq);
> > +       u32 num;
> > +
> > +       num = vq->packed_ring ? vq->packed.vring.num : vq->split.vring.num;
> > +
> > +       if (num != vq->vq.num_free)
> > +               return -EINVAL;
> >
>
> If we check this, I think we need to protect this with
> START_USE()/END_USE().

YES.


>
>
> > +
> > +       if (!vq->use_dma_api)
> > +               return -EINVAL;
> >
>
> Not a native spreak, but I think "dma_premapped" is better than "premapped"
> as "dma_premapped" implies "use_dma_api".

I am ok to fix this.

Thanks.


>
> Thanks
>
>
> > +
> > +       vq->premapped = true;
> > +
> > +       return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(virtqueue_set_premapped);
> > +
> >  /* Only available for split ring */
> >  struct virtqueue *vring_new_virtqueue(unsigned int index,
> >                                       unsigned int num,
> > diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> > index de6041deee37..2efd07b79ecf 100644
> > --- a/include/linux/virtio.h
> > +++ b/include/linux/virtio.h
> > @@ -78,6 +78,8 @@ bool virtqueue_enable_cb(struct virtqueue *vq);
> >
> >  unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
> >
> > +int virtqueue_set_premapped(struct virtqueue *_vq);
> > +
> >  bool virtqueue_poll(struct virtqueue *vq, unsigned);
> >
> >  bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
> > --
> > 2.32.0.3.g01195cf9f
> >
> >
>