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SubjectRe: [PATCH V2 net] Revert "vhost: cache used event for better performance"
On Thu, Jul 27, 2017 at 11:22:05AM +0800, Jason Wang wrote:
> This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92. Since it
> was reported to break vhost_net. We want to cache used event and use
> it to check for notification. The assumption was that guest won't move
> the event idx back, but this could happen in fact when 16 bit index
> wraps around after 64K entries.
>
> Signed-off-by: Jason Wang <jasowang@redhat.com>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
> - Changes from V1: tweak commit log
> - The patch is needed for -stable.
> ---
> drivers/vhost/vhost.c | 28 ++++++----------------------
> drivers/vhost/vhost.h | 3 ---
> 2 files changed, 6 insertions(+), 25 deletions(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index e4613a3..9cb3f72 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -308,7 +308,6 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> vq->avail = NULL;
> vq->used = NULL;
> vq->last_avail_idx = 0;
> - vq->last_used_event = 0;
> vq->avail_idx = 0;
> vq->last_used_idx = 0;
> vq->signalled_used = 0;
> @@ -1402,7 +1401,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
> r = -EINVAL;
> break;
> }
> - vq->last_avail_idx = vq->last_used_event = s.num;
> + vq->last_avail_idx = s.num;
> /* Forget the cached index value. */
> vq->avail_idx = vq->last_avail_idx;
> break;
> @@ -2241,6 +2240,10 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> __u16 old, new;
> __virtio16 event;
> bool v;
> + /* Flush out used index updates. This is paired
> + * with the barrier that the Guest executes when enabling
> + * interrupts. */
> + smp_mb();
>
> if (vhost_has_feature(vq, VIRTIO_F_NOTIFY_ON_EMPTY) &&
> unlikely(vq->avail_idx == vq->last_avail_idx))
> @@ -2248,10 +2251,6 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>
> if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
> __virtio16 flags;
> - /* Flush out used index updates. This is paired
> - * with the barrier that the Guest executes when enabling
> - * interrupts. */
> - smp_mb();
> if (vhost_get_avail(vq, flags, &vq->avail->flags)) {
> vq_err(vq, "Failed to get flags");
> return true;
> @@ -2266,26 +2265,11 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> if (unlikely(!v))
> return true;
>
> - /* We're sure if the following conditions are met, there's no
> - * need to notify guest:
> - * 1) cached used event is ahead of new
> - * 2) old to new updating does not cross cached used event. */
> - if (vring_need_event(vq->last_used_event, new + vq->num, new) &&
> - !vring_need_event(vq->last_used_event, new, old))
> - return false;
> -
> - /* Flush out used index updates. This is paired
> - * with the barrier that the Guest executes when enabling
> - * interrupts. */
> - smp_mb();
> -
> if (vhost_get_avail(vq, event, vhost_used_event(vq))) {
> vq_err(vq, "Failed to get used event idx");
> return true;
> }
> - vq->last_used_event = vhost16_to_cpu(vq, event);
> -
> - return vring_need_event(vq->last_used_event, new, old);
> + return vring_need_event(vhost16_to_cpu(vq, event), new, old);
> }
>
> /* This actually signals the guest, using eventfd. */
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index f720958..bb7c29b 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -115,9 +115,6 @@ struct vhost_virtqueue {
> /* Last index we used. */
> u16 last_used_idx;
>
> - /* Last used evet we've seen */
> - u16 last_used_event;
> -
> /* Used flags */
> u16 used_flags;
>
> --
> 2.7.4

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