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SubjectRe: [RFC][Patch v8 5/7] virtio: Enables to add a single descriptor to the host
On Mon, Feb 04, 2019 at 03:18:52PM -0500, Nitesh Narayan Lal wrote:
> This patch enables the caller to expose a single buffers to the
> other end using vring descriptor. It also allows the caller to
> perform this action in synchornous manner by using virtqueue_kick_sync.
>
> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>

I am not sure why do we need this API. Polling in guest
until host runs isn't great either since these
might be running on the same host CPU.



> ---
> drivers/virtio/virtio_ring.c | 72 ++++++++++++++++++++++++++++++++++++
> include/linux/virtio.h | 4 ++
> 2 files changed, 76 insertions(+)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index cd7e755484e3..93c161ac6a28 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -1695,6 +1695,52 @@ static inline int virtqueue_add(struct virtqueue *_vq,
> out_sgs, in_sgs, data, ctx, gfp);
> }
>
> +/**
> + * virtqueue_add_desc - add a buffer to a chain using a vring desc
> + * @vq: the struct virtqueue we're talking about.
> + * @addr: address of the buffer to add.
> + * @len: length of the buffer.
> + * @in: set if the buffer is for the device to write.
> + *
> + * Returns zero or a negative error (ie. ENOSPC, ENOMEM, EIO).
> + */
> +int virtqueue_add_desc(struct virtqueue *_vq, u64 addr, u32 len, int in)
> +{
> + struct vring_virtqueue *vq = to_vvq(_vq);
> + struct vring_desc *desc = vq->split.vring.desc;
> + u16 flags = in ? VRING_DESC_F_WRITE : 0;
> + unsigned int i;
> + void *data = (void *)addr;
> + int avail_idx;
> +
> + /* Sanity check */
> + if (!_vq)
> + return -EINVAL;
> +
> + START_USE(vq);
> + if (unlikely(vq->broken)) {
> + END_USE(vq);
> + return -EIO;
> + }
> +
> + i = vq->free_head;
> + flags &= ~VRING_DESC_F_NEXT;
> + desc[i].flags = cpu_to_virtio16(_vq->vdev, flags);
> + desc[i].addr = cpu_to_virtio64(_vq->vdev, addr);
> + desc[i].len = cpu_to_virtio32(_vq->vdev, len);
> +
> + vq->vq.num_free--;
> + vq->free_head = virtio16_to_cpu(_vq->vdev, desc[i].next);
> + vq->split.desc_state[i].data = data;
> + vq->split.avail_idx_shadow = 1;
> + avail_idx = vq->split.avail_idx_shadow;
> + vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev, avail_idx);
> + vq->num_added = 1;
> + END_USE(vq);
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_add_desc);
> +
> /**
> * virtqueue_add_sgs - expose buffers to other end
> * @vq: the struct virtqueue we're talking about.
> @@ -1842,6 +1888,32 @@ bool virtqueue_notify(struct virtqueue *_vq)
> }
> EXPORT_SYMBOL_GPL(virtqueue_notify);
>
> +/**
> + * virtqueue_kick_sync - update after add_buf and busy wait till update is done
> + * @vq: the struct virtqueue
> + *
> + * After one or more virtqueue_add_* calls, invoke this to kick
> + * the other side. Busy wait till the other side is done with the update.
> + *
> + * Caller must ensure we don't call this with other virtqueue
> + * operations at the same time (except where noted).
> + *
> + * Returns false if kick failed, otherwise true.
> + */
> +bool virtqueue_kick_sync(struct virtqueue *vq)
> +{
> + u32 len;
> +
> + if (likely(virtqueue_kick(vq))) {
> + while (!virtqueue_get_buf(vq, &len) &&
> + !virtqueue_is_broken(vq))
> + cpu_relax();
> + return true;
> + }
> + return false;
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_kick_sync);
> +
> /**
> * virtqueue_kick - update after add_buf
> * @vq: the struct virtqueue
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index fa1b5da2804e..58943a3a0e8d 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -57,6 +57,10 @@ int virtqueue_add_sgs(struct virtqueue *vq,
> unsigned int in_sgs,
> void *data,
> gfp_t gfp);
> +/* A desc with this init id is treated as an invalid desc */
> +int virtqueue_add_desc(struct virtqueue *_vq, u64 addr, u32 len, int in);
> +
> +bool virtqueue_kick_sync(struct virtqueue *vq);
>
> bool virtqueue_kick(struct virtqueue *vq);
>
> --
> 2.17.2

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