Re: [PATCH net-next RFC 3/5] vhost: introduce vhost_add_used_idx()

Michael S. Tsirkin
2017-09-26T19:13:09Z

On Fri, Sep 22, 2017 at 04:02:33PM +0800, Jason Wang wrote:
> This patch introduces a helper which just increase the used idx. This
> will be used in pair with vhost_prefetch_desc_indices() by batching
> code.
>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
> drivers/vhost/vhost.c | 33 +++++++++++++++++++++++++++++++++
> drivers/vhost/vhost.h | 1 +
> 2 files changed, 34 insertions(+)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 8424166d..6532cda 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2178,6 +2178,39 @@ int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int len)
> }
> EXPORT_SYMBOL_GPL(vhost_add_used);
>
> +int vhost_add_used_idx(struct vhost_virtqueue *vq, int n)
> +{
> + u16 old, new;
> +
> + old = vq->last_used_idx;
> + new = (vq->last_used_idx += n);
> + /* If the driver never bothers to signal in a very long while,
> + * used index might wrap around. If that happens, invalidate
> + * signalled_used index we stored. TODO: make sure driver
> + * signals at least once in 2^16 and remove this.
> + */
> + if (unlikely((u16)(new - vq->signalled_used) < (u16)(new - old)))
> + vq->signalled_used_valid = false;
> +
> + /* Make sure buffer is written before we update index. */
> + smp_wmb();
> + if (vhost_put_user(vq, cpu_to_vhost16(vq, vq->last_used_idx),
> + &vq->used->idx)) {
> + vq_err(vq, "Failed to increment used idx");
> + return -EFAULT;
> + }
> + if (unlikely(vq->log_used)) {
> + /* Log used index update. */
> + log_write(vq->log_base,
> + vq->log_addr + offsetof(struct vring_used, idx),
> + sizeof(vq->used->idx));
> + if (vq->log_ctx)
> + eventfd_signal(vq->log_ctx, 1);
> + }
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(vhost_add_used_idx);
> +
> static int __vhost_add_used_n(struct vhost_virtqueue *vq,
> struct vring_used_elem *heads,
> unsigned count)
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 16c2cb6..5dd6c05 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -199,6 +199,7 @@ int __vhost_get_vq_desc(struct vhost_virtqueue *vq,
> void vhost_discard_vq_desc(struct vhost_virtqueue *, int n);
>
> int vhost_vq_init_access(struct vhost_virtqueue *);
> +int vhost_add_used_idx(struct vhost_virtqueue *vq, int n);
> int vhost_add_used(struct vhost_virtqueue *, unsigned int head, int len);
> int vhost_add_used_n(struct vhost_virtqueue *, struct vring_used_elem *heads,
> unsigned count);

Please change the API to hide the fact that there's an index that needs
to be updated.

> --
> 2.7.4