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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 5 May 2011 12:38:17 +0300

On Wed, May 04, 2011 at 04:56:09PM -0500, Tom Lendacky wrote:
> 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

In practice it gives the same result because of the alignment, but sure.
Thanks!

-- 
MST