On 2019/10/22 下午2:48, Liu, Yong wrote:
Hi Jason,
My answers are inline.
-----Original Message-----
From: Jason Wang [mailto:jasowang@xxxxxxxxxx]
Sent: Tuesday, October 22, 2019 10:45 AM
To: Liu, Yong <yong.liu@xxxxxxxxx>; mst@xxxxxxxxxx; Bie, Tiwei
<tiwei.bie@xxxxxxxxx>
Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
Subject: Re: [PATCH] virtio_ring: fix packed ring event may missing
On 2019/10/22 上午1:10, Marvin Liu wrote:
When callback is delayed, virtio expect that vhost will kick when
rolling over event offset. Recheck should be taken as used index may
exceed event offset between status check and driver event update.
However, it is possible that flags was not modified if descriptors are
chained or in_order feature was negotiated. So flags at event offset
may not be valid for descriptor's status checking. Fix it by using last
used index as replacement. Tx queue will be stopped if there's not
enough freed buffers after recheck.
Signed-off-by: Marvin Liu <yong.liu@xxxxxxxxx>
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index bdc08244a648..a8041e451e9e 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -1499,9 +1499,6 @@ static bool
virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
* counter first before updating event flags.
*/
virtio_wmb(vq->weak_barriers);
- } else {
- used_idx = vq->last_used_idx;
- wrap_counter = vq->packed.used_wrap_counter;
}
if (vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DISABLE)
{
@@ -1518,7 +1515,9 @@ static bool
virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
*/
virtio_mb(vq->weak_barriers);
- if (is_used_desc_packed(vq, used_idx, wrap_counter)) {
+ if (is_used_desc_packed(vq,
+ vq->last_used_idx,
+ vq->packed.used_wrap_counter)) {
END_USE(vq);
return false;
}
Hi Marvin:
Two questions:
1) Do we support IN_ORDER in kernel driver?
Not support by now. But issue still can be possible if in_direct disabled and meanwhile descs are chained.
Due to packed ring desc status should check one by one, chose arbitrary position may cause issue.
Right, then it's better to mention IN_ORDER as future features.
2) Should we check IN_ORDER in this case otherwise we may end up with
interrupt storm when IN_ORDER is not negotiated?
Interrupt number will not increase here, event offset value calculated as before.
Here just recheck whether new used descs is enough for next around xmit.
If backend was slow, most likely Tx queue will sleep for a while until used index go over event offset.
Ok, but what if the backend is almost as fast as guest driver? E.g in
virtio-net we had:
if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
netif_stop_subqueue(dev, qnum);
if (!use_napi &&
unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
/* More just got used, free them then recheck. */
free_old_xmit_skbs(sq, false);
if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
netif_start_subqueue(dev, qnum);
virtqueue_disable_cb(sq->vq);
}
}
}
I worry that we may end up with toggling queue state in the case
(sq->vq->num_free is near 2 + MAX_SKB_FRAGS).
It looks to me the correct thing to implement is to calculate the head
descriptor of a chain that sits at 3/4.
Thanks
Thanks,
Marvin
Thanks
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