On Wed, May 17, 2023 at 4:54 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Thu, Apr 13, 2023 at 02:40:27PM +0800, Jason Wang wrote:
> > We used to busy waiting on the cvq command this tends to be
> > problematic since there no way for to schedule another process which
> > may serve for the control virtqueue. This might be the case when the
> > control virtqueue is emulated by software. This patch switches to use
> > completion to allow the CPU to sleep instead of busy waiting for the
> > cvq command.
> >
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > ---
> > Changes since V1:
> > - use completion for simplicity
> > - don't try to harden the CVQ command which requires more thought
> > Changes since RFC:
> > - break the device when timeout
> > - get buffer manually since the virtio core check more_used() instead
> > ---
> > drivers/net/virtio_net.c | 21 ++++++++++++++-------
> > 1 file changed, 14 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 2e56bbf86894..d3eb8fd6c9dc 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -19,6 +19,7 @@
> > #include <linux/average.h>
> > #include <linux/filter.h>
> > #include <linux/kernel.h>
> > +#include <linux/completion.h>
> > #include <net/route.h>
> > #include <net/xdp.h>
> > #include <net/net_failover.h>
> > @@ -295,6 +296,8 @@ struct virtnet_info {
> >
> > /* failover when STANDBY feature enabled */
> > struct failover *failover;
> > +
> > + struct completion completion;
> > };
> >
> > struct padded_vnet_hdr {
> > @@ -1709,6 +1712,13 @@ static bool try_fill_recv(struct virtnet_info *vi, struct receive_queue *rq,
> > return !oom;
> > }
> >
> > +static void virtnet_cvq_done(struct virtqueue *cvq)
> > +{
> > + struct virtnet_info *vi = cvq->vdev->priv;
> > +
> > + complete(&vi->completion);
> > +}
> > +
> > static void skb_recv_done(struct virtqueue *rvq)
> > {
> > struct virtnet_info *vi = rvq->vdev->priv;
> > @@ -2169,12 +2179,8 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
> > if (unlikely(!virtqueue_kick(vi->cvq)))
> > return vi->ctrl->status == VIRTIO_NET_OK;
> >
> > - /* Spin for a response, the kick causes an ioport write, trapping
> > - * into the hypervisor, so the request should be handled immediately.
> > - */
> > - while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> > - !virtqueue_is_broken(vi->cvq))
> > - cpu_relax();
> > + wait_for_completion(&vi->completion);
> > + virtqueue_get_buf(vi->cvq, &tmp);
> >
> > return vi->ctrl->status == VIRTIO_NET_OK;
>
> This seems to break surprise removal and other
> situations where vq gets broken since callbacks
> aren't usually invoked then.
Yes, so I think I can go back to the original idea by simply adding
cond_resched() here.
>
>
> > }
> > @@ -3672,7 +3678,7 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
> >
> > /* Parameters for control virtqueue, if any */
> > if (vi->has_cvq) {
> > - callbacks[total_vqs - 1] = NULL;
> > + callbacks[total_vqs - 1] = virtnet_cvq_done;
> > names[total_vqs - 1] = "control";
> > }
> >
>
> There is a cost to this, in that we are burning an extra MSI vector
> for the slow path cvq. if device has 3 vectors, suddenly we can't
> allocate vectors for rx and tx, big problem.
>
> So I'm afraid we need to pass a new flag that will share
> the config changed interrupt and cvq.
See above, it looks to me a simple cond_resched() is sufficient, then
we don't need a new vector.
Thanks
>
>
>
> > @@ -4122,6 +4128,7 @@ static int virtnet_probe(struct virtio_device *vdev)
> > if (vi->has_rss || vi->has_rss_hash_report)
> > virtnet_init_default_rss(vi);
> >
> > + init_completion(&vi->completion);
> > enable_rx_mode_work(vi);
> >
> > /* serialize netdev register + virtio_device_ready() with ndo_open() */
> > --
> > 2.25.1
>
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