Re: [PATCH vhost v10 02/10] virtio_ring: introduce virtqueue_set_premapped()

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

On Tue, 27 Jun 2023 16:03:23 +0800, Jason Wang <jasowang@redhat.com> wrote:
> On Fri, Jun 2, 2023 at 5:22 PM Xuan Zhuo <xuanzhuo@linux.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@linux.alibaba.com>
> > ---
> >  drivers/virtio/virtio_ring.c | 40 ++++++++++++++++++++++++++++++++++++
> >  include/linux/virtio.h       |  2 ++
> >  2 files changed, 42 insertions(+)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 72ed07a604d4..2afdfb9e3e30 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. */
> > @@ -2059,6 +2062,7 @@ 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;
> > @@ -2548,6 +2552,7 @@ 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;
> > @@ -2691,6 +2696,41 @@ 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. So the driver must call
> > + * virtqueue_get_buf_premapped()/virtqueue_detach_unused_buf_premapped().
> > + *
> > + * This must be called before adding any buf to vring.
>
> And any old buffer should be detached?

I mean that before adding any buf, So there are not old buffer.


>
> > + * So this should be called immediately after init vq or vq reset.
>
> Any way to detect and warn in this case? (not a must if it's too
> expensive to do the check)


I can try to check whether the qeueu is empty.


>
> > + *
> > + * 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);
> > +
> > +       if (!vq->use_dma_api)
> > +               return -EINVAL;
> > +
> > +       vq->premapped = true;
>
> I guess there should be a way to disable it. Would it be useful for
> the case when AF_XDP sockets were destroyed?

Yes.

When we reset the queue, the vq->premapped will be set to 0.

The is called after find_vqs or reset vq.

Thanks.



>
> Thanks
>
>
> > +
> > +       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 b93238db94e3..1fc0e1023bd4 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
> >
>