[Qemu-devel] [PATCHv2-RFC 2/2] virtio: publish used idx

[Michael S. Tsirkin]

Signed-off-by: Michael S. Tsirkin <address@hidden>
---
 drivers/net/virtio_net.c     |    2 ++
 drivers/virtio/virtio_ring.c |   17 +++++++++++++++--
 include/linux/virtio_ring.h  |    4 ++++
 3 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 78eb319..30e7483 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -23,6 +23,7 @@
 #include <linux/module.h>
 #include <linux/virtio.h>
 #include <linux/virtio_net.h>
+#include <linux/virtio_ring.h>
 #include <linux/scatterlist.h>
 #include <linux/if_vlan.h>
 #include <linux/slab.h>
@@ -1056,6 +1057,7 @@ static unsigned int features[] = {
        VIRTIO_NET_F_GUEST_ECN, VIRTIO_NET_F_GUEST_UFO,
        VIRTIO_NET_F_MRG_RXBUF, VIRTIO_NET_F_STATUS, VIRTIO_NET_F_CTRL_VQ,
        VIRTIO_NET_F_CTRL_RX, VIRTIO_NET_F_CTRL_VLAN,
+       VIRTIO_RING_F_PUBLISH_USED,
 };
 
 static struct virtio_driver virtio_net_driver = {
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 2241342..4a5458e 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -92,6 +92,9 @@ struct vring_virtqueue
        /* Last used index we've seen. */
        u16 last_used_idx;
 
+       /* Publish last used index we've seen at this location. */
+       u16 *publish_last_used_idx;
+
        /* How to notify other side. FIXME: commonalize hcalls! */
        void (*notify)(struct virtqueue *vq);
 
@@ -325,7 +328,7 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int 
*len)
        /* detach_buf clears data, so grab it now. */
        ret = vq->data[i];
        detach_buf(vq, i);
-       vq->last_used_idx++;
+       *vq->publish_last_used_idx = ++vq->last_used_idx;
        END_USE(vq);
        return ret;
 }
@@ -348,6 +351,8 @@ bool virtqueue_enable_cb(struct virtqueue *_vq)
        /* We optimistically turn back on interrupts, then check if there was
         * more to do. */
        vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
+       /* Besides flags write, this barrier also flushes out
+        * last available index write. */
        virtio_mb();
        if (unlikely(more_used(vq))) {
                END_USE(vq);
@@ -425,13 +430,19 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
        if (!vq)
                return NULL;
 
-       vring_init(&vq->vring, num, pages, vring_align, false);
+       vring_init(&vq->vring, num, pages, vring_align,
+                  virtio_has_feature(vdev, VIRTIO_RING_F_PUBLISH_USED));
+       if (virtio_has_feature(vdev, VIRTIO_RING_F_PUBLISH_USED))
+           vq->publish_last_used_idx = &vq->vring.avail->last_used_idx;
+       else
+           vq->publish_last_used_idx = &vq->last_used_idx;
        vq->vq.callback = callback;
        vq->vq.vdev = vdev;
        vq->vq.name = name;
        vq->notify = notify;
        vq->broken = false;
        vq->last_used_idx = 0;
+       *vq->publish_last_used_idx = 0;
        vq->num_added = 0;
        list_add_tail(&vq->vq.list, &vdev->vqs);
 #ifdef DEBUG
@@ -473,6 +484,8 @@ void vring_transport_features(struct virtio_device *vdev)
                switch (i) {
                case VIRTIO_RING_F_INDIRECT_DESC:
                        break;
+               case VIRTIO_RING_F_PUBLISH_USED:
+                       break;
                default:
                        /* We don't understand this bit. */
                        clear_bit(i, vdev->features);
diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index c5f3ee7..0de35c5 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -29,6 +29,9 @@
 /* We support indirect buffer descriptors */
 #define VIRTIO_RING_F_INDIRECT_DESC    28
 
+/* The Guest publishes last-seen used index at the end of the avail ring. */
+#define VIRTIO_RING_F_PUBLISH_USED     29
+
 /* Virtio ring descriptors: 16 bytes.  These can chain together via "next". */
 struct vring_desc {
        /* Address (guest-physical). */
@@ -50,6 +53,7 @@ struct vring_avail {
 struct vring_avail_ctrl {
        __u16 flags;
        __u16 idx;
+       __u16 last_used_idx;
 };
 
 /* u32 is used here for ids for padding reasons. */
-- 
1.7.1.12.g42b7f