On Wed, Dec 05, 2018 at 07:01:23PM +0300, Ilya Maximets wrote:
> On 28.11.2018 12:47, Maxime Coquelin wrote:
> > Instead of writing back descriptors chains in order, let's
> > write the first chain flags last in order to improve batching.
>
> I'm not sure if this fully compliant with virtio spec.
> It says that 'each side (driver and device) are only required to poll
> (or test) a single location in memory', but it does not forbid to
> test other descriptors. So, if the driver will try to check not only
> 'the next device descriptor after the one they processed previously,
> in circular order' but a few descriptors ahead, it could read an
> inconsistent memory because there are no more write barriers between
> updates for flags and id/len for them.
>
> What do you think ?
Write barriers for SMP effects are quite cheap on most architectures.
So adding them before each flag write is probably not a big deal.
> >
> > With Kernel's pktgen benchmark, ~3% performance gain is measured.
> >
> > Signed-off-by: Maxime Coquelin <maxime.coque...@redhat.com>
> > Tested-by: Jens Freimann <jfreim...@redhat.com>
> > Reviewed-by: Jens Freimann <jfreim...@redhat.com>
> > ---
> > lib/librte_vhost/virtio_net.c | 37 ++++++++++++++++++++++-------------
> > 1 file changed, 23 insertions(+), 14 deletions(-)
> >
> > diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
> > index 5e1a1a727..f54642c2d 100644
> > --- a/lib/librte_vhost/virtio_net.c
> > +++ b/lib/librte_vhost/virtio_net.c
> > @@ -135,19 +135,10 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
> > struct vhost_virtqueue *vq)
> > {
> > int i;
> > - uint16_t used_idx = vq->last_used_idx;
> > + uint16_t head_flags, head_idx = vq->last_used_idx;
> >
> > - /* Split loop in two to save memory barriers */
> > - for (i = 0; i < vq->shadow_used_idx; i++) {
> > - vq->desc_packed[used_idx].id = vq->shadow_used_packed[i].id;
> > - vq->desc_packed[used_idx].len = vq->shadow_used_packed[i].len;
> > -
> > - used_idx += vq->shadow_used_packed[i].count;
> > - if (used_idx >= vq->size)
> > - used_idx -= vq->size;
> > - }
> > -
> > - rte_smp_wmb();
> > + if (unlikely(vq->shadow_used_idx == 0))
> > + return;
> >
> > for (i = 0; i < vq->shadow_used_idx; i++) {
> > uint16_t flags;
> > @@ -165,12 +156,22 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
> > flags &= ~VRING_DESC_F_AVAIL;
> > }
> >
> > - vq->desc_packed[vq->last_used_idx].flags = flags;
> > + vq->desc_packed[vq->last_used_idx].id =
> > + vq->shadow_used_packed[i].id;
> > + vq->desc_packed[vq->last_used_idx].len =
> > + vq->shadow_used_packed[i].len;
> > +
> > + if (i > 0) {
Specifically here?
> > + vq->desc_packed[vq->last_used_idx].flags = flags;
> >
> > - vhost_log_cache_used_vring(dev, vq,
> > + vhost_log_cache_used_vring(dev, vq,
> > vq->last_used_idx *
> > sizeof(struct vring_packed_desc),
> > sizeof(struct vring_packed_desc));
> > + } else {
> > + head_idx = vq->last_used_idx;
> > + head_flags = flags;
> > + }
> >
> > vq->last_used_idx += vq->shadow_used_packed[i].count;
> > if (vq->last_used_idx >= vq->size) {
> > @@ -180,7 +181,15 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
> > }
> >
> > rte_smp_wmb();
> > +
> > + vq->desc_packed[head_idx].flags = head_flags;
> > vq->shadow_used_idx = 0;
> > +
> > + vhost_log_cache_used_vring(dev, vq,
> > + head_idx *
> > + sizeof(struct vring_packed_desc),
> > + sizeof(struct vring_packed_desc));
> > +
> > vhost_log_cache_sync(dev, vq);
> > }
> >
> >
