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Subject[RFC v6 4/5] virtio_ring: add event idx support in packed ring
Date
This commit introduces the EVENT_IDX support in packed ring.

Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
---
drivers/virtio/virtio_ring.c | 74 ++++++++++++++++++++++++++++++++----
1 file changed, 67 insertions(+), 7 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 983ce5ffda1b..aefd7ac40928 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -1048,7 +1048,7 @@ 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;

@@ -1057,9 +1057,19 @@ 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->avail_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(),
@@ -1068,7 +1078,10 @@ 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;
}
@@ -1177,6 +1190,15 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
ret = vq->desc_state_packed[id].data;
detach_buf_packed(vq, id, ctx);

+ /* 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 |
+ ((u16)vq->used_wrap_counter << 15)));
+
#ifdef DEBUG
vq->last_add_time_valid = false;
#endif
@@ -1204,9 +1226,20 @@ static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)

/* We optimistically turn back on interrupts, then check if there was
* more to do. */
+ /* Depending on the VIRTIO_RING_F_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. */
+
+ vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
+ vq->last_used_idx |
+ ((u16)vq->used_wrap_counter << 15));

if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
- vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
+ /* We need to update event offset and event wrap
+ * counter first before updating event flags. */
+ virtio_wmb(vq->weak_barriers);
+ 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);
}
@@ -1232,21 +1265,48 @@ 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_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->used_wrap_counter;
+
+ used_idx = vq->last_used_idx + bufs;
+ if (used_idx >= vq->vring_packed.num) {
+ used_idx -= vq->vring_packed.num;
+ wrap_counter ^= 1;
+ }
+
+ 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) {
- vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
+ /* We need to update event offset and event wrap
+ * counter first before updating event flags. */
+ virtio_wmb(vq->weak_barriers);
+ 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);
- /* We need to enable interrupts first before re-checking
- * for more used buffers. */
- virtio_mb(vq->weak_barriers);
}

+ /* We need to update event suppression structure first
+ * before re-checking for more used buffers. */
+ virtio_mb(vq->weak_barriers);
+
if (more_used_packed(vq)) {
END_USE(vq);
return false;
--
2.17.0
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