netdev - Re: [PATCH vhost v4 10/10] virtio_ring: virtqueue_set_dma

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Message-ID: <1711009281.7778504-3-xuanzhuo@linux.alibaba.com>
Date: Thu, 21 Mar 2024 16:21:21 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: Jason Wang <jasowang@...hat.com>
Cc: virtualization@...ts.linux.dev,
 "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>,
 netdev@...r.kernel.org
Subject: Re: [PATCH vhost v4 10/10] virtio_ring: virtqueue_set_dma_premapped support disable

On Thu, 21 Mar 2024 14:02:14 +0800, Jason Wang <jasowang@...hat.com> wrote:
> On Tue, Mar 12, 2024 at 11:36 AM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> >
> > Now, the API virtqueue_set_dma_premapped just support to
> > enable premapped mode.
> >
> > If we allow enabling the premapped dynamically, we should
> > make this API to support disable the premapped mode.
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > ---
> >  drivers/virtio/virtio_ring.c | 34 ++++++++++++++++++++++++++--------
> >  include/linux/virtio.h       |  2 +-
> >  2 files changed, 27 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 34f4b2c0c31e..3bf69cae4965 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -2801,6 +2801,7 @@ EXPORT_SYMBOL_GPL(virtqueue_resize);
> >  /**
> >   * virtqueue_set_dma_premapped - set the vring premapped mode
> >   * @_vq: the struct virtqueue we're talking about.
> > + * @premapped: enable/disable the premapped mode.
> >   *
> >   * Enable the premapped mode of the vq.
> >   *
> > @@ -2819,9 +2820,10 @@ EXPORT_SYMBOL_GPL(virtqueue_resize);
> >   * 0: success.
> >   * -EINVAL: vring does not use the dma api, so we can not enable premapped mode.
> >   */
> > -int virtqueue_set_dma_premapped(struct virtqueue *_vq)
> > +int virtqueue_set_dma_premapped(struct virtqueue *_vq, bool premapped)
>
> I think we need to document the requirement for calling this.
>
> Looking at the code, it seems it requires to stop the datapath and
> detach all the used buffers?

YES. The complete document is:

/**
 * virtqueue_set_dma_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.
 */

Thanks

>
> Thanks
>