netdev - Re: virtio_net and SMP guests

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Message-Id: <200801101651.58908.borntraeger@de.ibm.com>
Date:	Thu, 10 Jan 2008 16:51:58 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	virtualization@...ts.linux-foundation.org
Cc:	kvm-devel <kvm-devel@...ts.sourceforge.net>,
	Anthony Liguori <aliguori@...ibm.com>,
	Dor Laor <dor.laor@...ranet.com>, netdev@...r.kernel.org
Subject: Re: virtio_net and SMP guests

Am Donnerstag, 10. Januar 2008 schrieb Christian Borntraeger:
> Am Donnerstag, 10. Januar 2008 schrieb Christian Borntraeger:
> > Am Dienstag, 18. Dezember 2007 schrieb Rusty Russell:
> > > To me this points to doing interrupt suppression a different way.  If 
we
> > > have a ->disable_cb() virtio function, and call it before we call
> > > netif_rx_schedule, does that fix it?
> > 
> > The fix looks good and I agree with it.
> > 
> > There is one problem that I try to find for some days, but the following 
> > BUG_ON triggers:
> > 
> > static void vring_disable_cb(struct virtqueue *_vq)
> > {
> >         struct vring_virtqueue *vq = to_vvq(_vq);
> > 
> >         START_USE(vq);
> > ---->   BUG_ON(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT);
> >         vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
> >         END_USE(vq);
> > }
> 
> Ok, I found it:
> 
> static int virtnet_open(struct net_device *dev)
> {
>         struct virtnet_info *vi = netdev_priv(dev);
>         try_fill_recv(vi);
>         /* If we didn't even get one input buffer, we're useless. */
>         if (vi->num == 0)
>                 return -ENOMEM;
> 					---> int for new packet
> 					static void skb_recv_done(struct virtqueue *rvq)
> 					{
>         					struct virtnet_info *vi = rvq->vdev->priv;
> 					        /* Suppress further interrupts. */
> 					        rvq->vq_ops->disable_cb(rvq);
> 					        netif_rx_schedule(vi->dev, &vi->napi);
> 					}
> 					- poll is not yet possible, no softirq
> 					- return from interrupt
>         napi_enable(&vi->napi);
> 	vi->rvq->vq_ops->disable_cb(vi->rvq);
> 					---> BUG: its already disabled

Btw. this problem also happens on single processor guests.

What about the following patch:

---
 drivers/net/virtio_net.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

Index: kvm/drivers/net/virtio_net.c
===================================================================
--- kvm.orig/drivers/net/virtio_net.c
+++ kvm/drivers/net/virtio_net.c
@@ -179,9 +179,12 @@ static void try_fill_recv(struct virtnet
 static void skb_recv_done(struct virtqueue *rvq)
 {
 	struct virtnet_info *vi = rvq->vdev->priv;
-	/* Suppress further interrupts. */
-	rvq->vq_ops->disable_cb(rvq);
-	netif_rx_schedule(vi->dev, &vi->napi);
+	/* Schedule NAPI, Suppress further interrupts if successful. */
+
+	if (netif_rx_schedule_prep(vi->dev, &vi->napi)) {
+		rvq->vq_ops->disable_cb(rvq);
+		__netif_rx_schedule(vi->dev, &vi->napi);
+	}
 }
 
 static int virtnet_poll(struct napi_struct *napi, int budget)
--
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