Re: [RFC v3 4/5] virtio_ring: add event idx support in packed ring
From: Jason Wang
Date: Wed May 16 2018 - 01:01:21 EST
On 2018å04æ25æ 13:15, Tiwei Bie wrote:
This commit introduces the event idx support in packed
ring. This feature is temporarily disabled, because the
implementation in this patch may not work as expected,
and some further discussions on the implementation are
needed, e.g. do we have to check the wrap counter when
checking whether a kick is needed?
Signed-off-by: Tiwei Bie <tiwei.bie@xxxxxxxxx>
---
drivers/virtio/virtio_ring.c | 53 ++++++++++++++++++++++++++++++++++++++++----
1 file changed, 49 insertions(+), 4 deletions(-)
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 0181e93897be..b1039c2985b9 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -986,7 +986,7 @@ 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;
bool needs_kick;
u32 snapshot;
@@ -995,7 +995,12 @@ 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, snapshot & 0xffff);
flags = cpu_to_virtio16(_vq->vdev, snapshot >> 16) & 0x3;
#ifdef DEBUG
@@ -1006,7 +1011,10 @@ 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(off_wrap & ~(1<<15), new, old);
+ else
+ needs_kick = (flags != VRING_EVENT_F_DISABLE);
END_USE(vq);
return needs_kick;
}
@@ -1116,6 +1124,15 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
if (vq->last_used_idx >= vq->vring_packed.num)
vq->last_used_idx -= vq->vring_packed.num;
+ /* 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 |
+ (vq->wrap_counter << 15)));
+
#ifdef DEBUG
vq->last_add_time_valid = false;
#endif
@@ -1143,10 +1160,17 @@ 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_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. */
+
+ vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
+ vq->last_used_idx | (vq->wrap_counter << 15));
Using vq->wrap_counter seems not correct, what we need is the warp
counter for the last_used_idx not next_avail_idx.
And I think there's even no need to bother with event idx here, how
about just set VRING_EVENT_F_ENABLE?
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);
}
@@ -1172,15 +1196,34 @@ 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 */
+ bufs = (u16)(vq->next_avail_idx - vq->last_used_idx) * 3 / 4;
bufs could be more than vq->num here, is this intended?
+
+ used_idx = vq->last_used_idx + bufs;
+ wrap_counter = vq->wrap_counter;
+
+ if (used_idx >= vq->vring_packed.num) {
+ used_idx -= vq->vring_packed.num;
+ wrap_counter ^= 1;
When used_idx is greater or equal vq->num, there's no need to flip
warp_counter bit since it should match next_avail_idx.
And we need also care about the case when next_avail wraps but used_idx
not. so we probaly need:
else if (vq->next_avail_idx < used_idx) {
ÂÂÂ wrap_counter ^= 1;
}
I think maybe it's time to add some sample codes in the spec to avoid
duplicating the efforts(bugs).
Thanks
+ }
+
+ 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);
}
@@ -1822,8 +1865,10 @@ 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
case VIRTIO_F_VERSION_1:
break;
case VIRTIO_F_IOMMU_PLATFORM: