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SubjectRe: [virtio-dev] Re: [PATCH net-next v2 4/5] virtio_ring: add event idx support in packed ring
On Fri, Sep 07, 2018 at 10:10:14AM -0400, Michael S. Tsirkin wrote:
> On Wed, Jul 11, 2018 at 10:27:10AM +0800, Tiwei Bie wrote:
> > This commit introduces the EVENT_IDX support in packed ring.
> >
> > Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
>
> Besides the usual comment about hard-coded constants like <<15:
> does this actually do any good for performance? We don't
> have to if we do not want to.

Got it. Thanks.

>
> > ---
> > drivers/virtio/virtio_ring.c | 73 ++++++++++++++++++++++++++++++++----
> > 1 file changed, 65 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index f317b485ba54..f79a1e17f7d1 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -1050,7 +1050,7 @@ 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, wrap_counter, event_idx;
> > bool needs_kick;
> > u32 snapshot;
> >
> > @@ -1059,9 +1059,19 @@ 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 = READ_ONCE(*(u32 *)vq->vring_packed.device);
> > + off_wrap = virtio16_to_cpu(_vq->vdev, (__virtio16)(snapshot & 0xffff));
> > flags = virtio16_to_cpu(_vq->vdev, (__virtio16)(snapshot >> 16)) & 0x3;
>
>
> some kind of struct union would be helpful to make this readable.

I will define a struct/union for this.

>
> >
> > + wrap_counter = off_wrap >> 15;
> > + event_idx = off_wrap & ~(1 << 15);
> > + if (wrap_counter != vq->avail_wrap_counter)
> > + event_idx -= vq->vring_packed.num;
> > +
> > #ifdef DEBUG
> > if (vq->last_add_time_valid) {
> > WARN_ON(ktime_to_ms(ktime_sub(ktime_get(),
> > @@ -1070,7 +1080,10 @@ 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(event_idx, new, old);
> > + else
> > + needs_kick = (flags != VRING_EVENT_F_DISABLE);
> > END_USE(vq);
> > return needs_kick;
> > }
> > @@ -1185,6 +1198,15 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
> > ret = vq->desc_state_packed[id].data;
> > detach_buf_packed(vq, id, ctx);
> >
> > + /* 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 |
> > + ((u16)vq->used_wrap_counter << 15)));
> > +
> > #ifdef DEBUG
> > vq->last_add_time_valid = false;
> > #endif
> > @@ -1213,8 +1235,18 @@ static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
> > /* We optimistically turn back on interrupts, then check if there was
> > * more to do. */
> >
> > + if (vq->event) {
> > + vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
> > + vq->last_used_idx |
> > + ((u16)vq->used_wrap_counter << 15));
> > + /* We need to update event offset and event wrap
> > + * counter first before updating event flags. */
> > + virtio_wmb(vq->weak_barriers);
> > + }
> > +
> > if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> > - 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);
> > }
> > @@ -1238,22 +1270,47 @@ static bool virtqueue_poll_packed(struct virtqueue *_vq, unsigned off_wrap)
> > 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. */
> >
> > + if (vq->event) {
> > + /* TODO: tune this threshold */
> > + bufs = (vq->vring_packed.num - _vq->num_free) * 3 / 4;
> > + wrap_counter = vq->used_wrap_counter;
> > +
> > + used_idx = vq->last_used_idx + bufs;
> > + 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));
> > +
> > + /* We need to update event offset and event wrap
> > + * counter first before updating event flags. */
> > + virtio_wmb(vq->weak_barriers);
> > + } else {
> > + used_idx = vq->last_used_idx;
> > + wrap_counter = vq->used_wrap_counter;
> > + }
> > +
> > if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> > - 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);
> > - /* We need to enable interrupts first before re-checking
> > - * for more used buffers. */
> > - virtio_mb(vq->weak_barriers);
> > }
> >
> > - if (more_used_packed(vq)) {
> > + /* We need to update event suppression structure first
> > + * before re-checking for more used buffers. */
> > + virtio_mb(vq->weak_barriers);
> > +
>
> mb is expensive. We should not do it if we changed nothing.

I will try to avoid it when possible.

>
> > + if (is_used_desc_packed(vq, used_idx, wrap_counter)) {
> > END_USE(vq);
> > return false;
> > }
> > --
> > 2.18.0
>
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