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 <mst@xxxxxxxxxx>
> > ---
> > 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
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