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Re: [RFC v4 4/5] virtio_ring: add event idx support in packed ring






On 2018年05月16日 16:37, Tiwei Bie wrote:
This commit introduces the event idx support in
packed ring.

Signed-off-by: Tiwei Bie <tiwei.bie@xxxxxxxxx>
---
  drivers/virtio/virtio_ring.c | 75 +++++++++++++++++++++++++++++++++---
  1 file changed, 70 insertions(+), 5 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index c6c5deb0e3ae..de3839f3621a 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -1006,7 +1006,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
  static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
  {
  	struct vring_virtqueue *vq = to_vvq(_vq);
-	u16 flags;
+	u16 new, old, off_wrap, flags, wrap_counter, event_idx;
  	bool needs_kick;
  	u32 snapshot;
@@ -1015,9 +1015,19 @@ static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
  	 * suppressions. */
  	virtio_mb(vq->weak_barriers);
+ old = vq->next_avail_idx - vq->num_added;
+	new = vq->next_avail_idx;
+	vq->num_added = 0;
+
  	snapshot = *(u32 *)vq->vring_packed.device;
+	off_wrap = virtio16_to_cpu(_vq->vdev, (__virtio16)(snapshot & 0xffff));
  	flags = virtio16_to_cpu(_vq->vdev, (__virtio16)(snapshot >> 16)) & 0x3;
+ wrap_counter = off_wrap >> 15;
+	event_idx = off_wrap & ~(1<<15);
+	if (wrap_counter != vq->wrap_counter)
+		event_idx -= vq->vring_packed.num;
+
  #ifdef DEBUG
  	if (vq->last_add_time_valid) {
  		WARN_ON(ktime_to_ms(ktime_sub(ktime_get(),
@@ -1026,7 +1036,10 @@ static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
  	vq->last_add_time_valid = false;
  #endif
- needs_kick = (flags != VRING_EVENT_F_DISABLE);
+	if (flags == VRING_EVENT_F_DESC)
+		needs_kick = vring_need_event(event_idx, new, old);
+	else
+		needs_kick = (flags != VRING_EVENT_F_DISABLE);
  	END_USE(vq);
  	return needs_kick;
  }
@@ -1098,7 +1111,7 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
  					  void **ctx)
  {
  	struct vring_virtqueue *vq = to_vvq(_vq);
-	u16 last_used, id;
+	u16 wrap_counter, last_used, id;
  	void *ret;
START_USE(vq);
@@ -1138,6 +1151,19 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
  	ret = vq->desc_state[id].data;
  	detach_buf_packed(vq, last_used, id, ctx);
+ wrap_counter = vq->wrap_counter;
+	if (vq->last_used_idx > vq->next_avail_idx)
+		wrap_counter ^= 1;
+
+	/* If we expect an interrupt for the next entry, tell host
+	 * by writing event index and flush out the write before
+	 * the read in the next get_buf call. */
+	if (vq->event_flags_shadow == VRING_EVENT_F_DESC)
+		virtio_store_mb(vq->weak_barriers,
+				&vq->vring_packed.driver->off_wrap,
+				cpu_to_virtio16(_vq->vdev, vq->last_used_idx |
+						(wrap_counter << 15)));
+
  #ifdef DEBUG
  	vq->last_add_time_valid = false;
  #endif
@@ -1160,15 +1186,27 @@ static void virtqueue_disable_cb_packed(struct virtqueue *_vq)
  static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
  {
  	struct vring_virtqueue *vq = to_vvq(_vq);
+	u16 wrap_counter;
START_USE(vq); /* We optimistically turn back on interrupts, then check if there was
  	 * more to do. */
+	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
+	 * either clear the flags bit or point the event index at the next
+	 * entry. Always update the event index to keep code simple. */
+
+	wrap_counter = vq->wrap_counter;
+	if (vq->last_used_idx > vq->next_avail_idx)

Should this be ">=" consider rx refill may try to completely fill the ring?

+		wrap_counter ^= 1;
+
+	vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
+			vq->last_used_idx | (wrap_counter << 15));
if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
  		virtio_wmb(vq->weak_barriers);
-		vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
+		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC :
+						     VRING_EVENT_F_ENABLE;
  		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
  							vq->event_flags_shadow);
  	}
@@ -1194,15 +1232,40 @@ static bool virtqueue_poll_packed(struct virtqueue *_vq, unsigned last_used_idx)
  static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
  {
  	struct vring_virtqueue *vq = to_vvq(_vq);
+	u16 bufs, used_idx, wrap_counter;
START_USE(vq); /* We optimistically turn back on interrupts, then check if there was
  	 * more to do. */
+	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
+	 * either clear the flags bit or point the event index at the next
+	 * entry. Always update the event index to keep code simple. */
+
+	/* TODO: tune this threshold */
+	if (vq->next_avail_idx < vq->last_used_idx)
+		bufs = (vq->vring_packed.num + vq->next_avail_idx -
+				vq->last_used_idx) * 3 / 4;
+	else
+		bufs = (vq->next_avail_idx - vq->last_used_idx) * 3 / 4;
+
+	wrap_counter = vq->wrap_counter;
+	if (vq->last_used_idx > vq->next_avail_idx)
+		wrap_counter ^= 1;
+
+	used_idx = vq->last_used_idx + bufs;
+	if (used_idx >= vq->vring_packed.num) {
+		used_idx -= vq->vring_packed.num;
+		wrap_counter ^= 1;
+	}

Looks correct but maybe it's better to add some comments for such logic.

+
+	vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
+			used_idx | (wrap_counter << 15));
if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
  		virtio_wmb(vq->weak_barriers);
-		vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
+		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC :
+						     VRING_EVENT_F_ENABLE;
  		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
  							vq->event_flags_shadow);
  	}
@@ -1869,8 +1932,10 @@ void vring_transport_features(struct virtio_device *vdev)
  		switch (i) {
  		case VIRTIO_RING_F_INDIRECT_DESC:
  			break;
+#if 0
  		case VIRTIO_RING_F_EVENT_IDX:
  			break;
+#endif

Maybe it's time to remove this #if 0.

Thanks

  		case VIRTIO_F_VERSION_1:
  			break;
  		case VIRTIO_F_IOMMU_PLATFORM: