Subject: Re: [PATCH 05/18] virtio: used event index interface

From: Tom Lendacky <[email-masked]>
Date: Wed, 4 May 2011 16:56:09 -0500

On Wednesday, May 04, 2011 03:51:09 PM Michael S. Tsirkin wrote:
> Define a new feature bit for the guest to utilize a used_event index
> (like Xen) instead if a flag bit to enable/disable interrupts.
> 
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> ---
>  include/linux/virtio_ring.h |    9 +++++++++
>  1 files changed, 9 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> index e4d144b..f5c1b75 100644
> --- a/include/linux/virtio_ring.h
> +++ b/include/linux/virtio_ring.h
> @@ -29,6 +29,10 @@
>  /* We support indirect buffer descriptors */
>  #define VIRTIO_RING_F_INDIRECT_DESC	28
> 
> +/* The Guest publishes the used index for which it expects an interrupt
> + * at the end of the avail ring. Host should ignore the avail->flags
> field. */ +#define VIRTIO_RING_F_USED_EVENT_IDX	29
> +
>  /* Virtio ring descriptors: 16 bytes.  These can chain together via
> "next". */ struct vring_desc {
>  	/* Address (guest-physical). */
> @@ -83,6 +87,7 @@ struct vring {
>   *	__u16 avail_flags;
>   *	__u16 avail_idx;
>   *	__u16 available[num];
> + *	__u16 used_event_idx;
>   *
>   *	// Padding to the next align boundary.
>   *	char pad[];
> @@ -93,6 +98,10 @@ struct vring {
>   *	struct vring_used_elem used[num];
>   * };
>   */
> +/* We publish the used event index at the end of the available ring.
> + * It is at the end for backwards compatibility. */
> +#define vring_used_event(vr) ((vr)->avail->ring[(vr)->num])
> +
>  static inline void vring_init(struct vring *vr, unsigned int num, void *p,
>  			      unsigned long align)
>  {

You should update the vring_size procedure to account for the extra field at 
the end of the available ring by change the "(2 + num)" to "(3 + num)":
    return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (3 + num)

Tom