netdev - Re: [PATCH vhost 01/22] virtio_ring: virtqueue_set_dma

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Message-ID: <1697102334.7060938-2-xuanzhuo@linux.alibaba.com>
Date: Thu, 12 Oct 2023 17:18:54 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: virtualization@...ts.linux-foundation.org,
 "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>,
 Jason Wang <jasowang@...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 01/22] virtio_ring: virtqueue_set_dma_premapped support disable

On Thu, 12 Oct 2023 05:15:52 -0400, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> On Wed, Oct 11, 2023 at 05:27:07PM +0800, Xuan Zhuo wrote:
> > virtqueue_set_dma_premapped() adds a new parameter to disable the
> > virtqueue premapped mode.
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > ---
> >  drivers/net/virtio_net.c     |  2 +-
> >  drivers/virtio/virtio_ring.c | 11 ++++++++---
> >  include/linux/virtio.h       |  2 +-
> >  3 files changed, 10 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index fe7f314d65c9..6b5f47ebf9b2 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -737,7 +737,7 @@ static void virtnet_rq_set_premapped(struct virtnet_info *vi)
> >  		return;
> >
> >  	for (i = 0; i < vi->max_queue_pairs; i++) {
> > -		if (virtqueue_set_dma_premapped(vi->rq[i].vq))
> > +		if (virtqueue_set_dma_premapped(vi->rq[i].vq, true))
> >  			continue;
> >
> >  		vi->rq[i].do_dma = true;
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 51d8f3299c10..b3ded56722f4 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -2784,7 +2784,7 @@ 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 mode)
> >  {
> >  	struct vring_virtqueue *vq = to_vvq(_vq);
> >  	u32 num;
> > @@ -2803,8 +2803,13 @@ int virtqueue_set_dma_premapped(struct virtqueue *_vq)
> >  		return -EINVAL;
> >  	}
> >
> > -	vq->premapped = true;
> > -	vq->do_unmap = false;
> > +	if (mode) {
> > +		vq->premapped = true;
> > +		vq->do_unmap = false;
> > +	} else {
> > +		vq->premapped = false;
> > +		vq->do_unmap = vq->use_dma_api;
> > +	}
> >
> >  	END_USE(vq);
> >
> > diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> > index 4cc614a38376..1cf7b004348b 100644
> > --- a/include/linux/virtio.h
> > +++ b/include/linux/virtio.h
> > @@ -81,7 +81,7 @@ bool virtqueue_enable_cb(struct virtqueue *vq);
> >
> >  unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
> >
> > -int virtqueue_set_dma_premapped(struct virtqueue *_vq);
> > +int virtqueue_set_dma_premapped(struct virtqueue *_vq, bool mode);
> >
> >  bool virtqueue_poll(struct virtqueue *vq, unsigned);
>
> Wait a sec I thought we never change premapped. If you make this
> dynamic don't you need a bunch of locking?
> Or maybe queue is empty when you change this?
> If yes pls add a bunch of BUG_ON checks to make sure this is not misused.


Actually, this api is called immediately after the vq init or vq reset.

We already have such a check.

Thanks.

/**
 * 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.
 */
int virtqueue_set_dma_premapped(struct virtqueue *_vq, bool mode)
{
	struct vring_virtqueue *vq = to_vvq(_vq);
	u32 num;

	START_USE(vq);

	num = vq->packed_ring ? vq->packed.vring.num : vq->split.vring.num;

-->	if (num != vq->vq.num_free) {
		END_USE(vq);
		return -EINVAL;
	}

	if (!vq->use_dma_api) {
		END_USE(vq);
		return -EINVAL;
	}

	if (mode) {
		vq->premapped = true;
		vq->do_unmap = false;
	} else {
		vq->premapped = false;
		vq->do_unmap = vq->use_dma_api;
	}

	END_USE(vq);

	return 0;
}
EXPORT_SYMBOL_GPL(virtqueue_set_dma_premapped);


>
>
> > --
> > 2.32.0.3.g01195cf9f
>