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Message-ID: <915efeaa-4e47-9004-a722-54b0c40ebcbb@redhat.com>
Date: Wed, 16 May 2018 13:01:04 +0800
From: Jason Wang <jasowang@...hat.com>
To: Tiwei Bie <tiwei.bie@...el.com>, mst@...hat.com,
virtualization@...ts.linux-foundation.org,
linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Cc: wexu@...hat.com, jfreimann@...hat.com
Subject: Re: [RFC v3 4/5] virtio_ring: add event idx support in packed ring
On 2018年04月25日 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@...el.com>
> ---
> 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) {
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:
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