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SubjectRe: [RFC v3 4/5] virtio_ring: add event idx support in packed ring
On Wed, May 02, 2018 at 10:51:06AM +0800, Jason Wang wrote:
> 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@intel.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);
>
> I wonder whether or not the math is correct. Both new and event are in the
> unit of descriptor ring size, but old looks not.

What vring_need_event() cares is the distance between
`new` and `old`, i.e. vq->num_added. So I think there
is nothing wrong with `old`. But the calculation of the
distance between `new` and `event_idx` isn't right when
`new` wraps. How do you think about the below code:

wrap_counter = off_wrap >> 15;
event_idx = off_wrap & ~(1<<15);
if (wrap_counter != vq->wrap_counter)
event_idx -= vq->vring_packed.num;

needs_kick = vring_need_event(event_idx, new, old);

Best regards,
Tiwei Bie


>
> Thanks
>
> > + 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));
> > 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;
> > +
> > + 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;
> > + }
> > +
> > + 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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