Subject: Re: [PATCH net-next] virtio_net: Prevent misidentified spurious interrupts from killing the irq

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 2 Aug 2024 09:08:09 -0400

On Fri, Aug 02, 2024 at 03:42:06PM +0800, Heng Qi wrote:
> On Thu, 1 Aug 2024 10:04:05 -0400, "Michael S. Tsirkin" <[email-masked]> wrote:
> > On Thu, Aug 01, 2024 at 09:56:39PM +0800, Heng Qi wrote:
> > > Michael has effectively reduced the number of spurious interrupts in
> > > commit a7766ef18b33 ("virtio_net: disable cb aggressively") by disabling
> > > irq callbacks before cleaning old buffers.
> > > 
> > > But it is still possible that the irq is killed by mistake:
> > > 
> > >   When a delayed tx interrupt arrives, old buffers has been cleaned in
> > >   other paths (start_xmit and virtnet_poll_cleantx), then the interrupt is
> > >   mistakenly identified as a spurious interrupt in vring_interrupt.
> > > 
> > >   We should refrain from labeling it as a spurious interrupt; otherwise,
> > >   note_interrupt may inadvertently kill the legitimate irq.
> > > 
> > > Signed-off-by: Heng Qi <[email-masked]>
> > 
> > 
> > Is this a practical or theoretical issue? Do you observe an issue
> > and see that this patch fixes it? Or is this from code review?
> 
> 
> This issue was previously documented in our bugzilla, but that was in 2020.
> 
> I can't easily reproduce the issue after a7766ef18b33, but interrupt suppression
> under virtio is unreliable and out of sync, which is still a potential risk for
> DPUs where the VM and the device are not on the same host.
> 
> Thanks.

I find it hard to believe there's a real problem after a7766ef18b33.

> > > ---
> > >  drivers/net/virtio_net.c     |  9 ++++++
> > >  drivers/virtio/virtio_ring.c | 53 ++++++++++++++++++++++++++++++++++++
> > >  include/linux/virtio.h       |  3 ++
> > >  3 files changed, 65 insertions(+)
> > > 
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 0383a3e136d6..6d8739418203 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -2769,6 +2769,7 @@ static void virtnet_poll_cleantx(struct receive_queue *rq, int budget)
> > >  		do {
> > >  			virtqueue_disable_cb(sq->vq);
> > >  			free_old_xmit(sq, txq, !!budget);
> > > +			virtqueue_set_tx_oldbuf_cleaned(sq->vq, true);
> > >  		} while (unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
> > >  
> > >  		if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS) {
> > > @@ -3035,6 +3036,9 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> > >  
> > >  		free_old_xmit(sq, txq, false);
> > >  
> > > +		if (use_napi)
> > > +			virtqueue_set_tx_oldbuf_cleaned(sq->vq, true);
> > > +
> > >  	} while (use_napi && !xmit_more &&
> > >  	       unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
> > >  
> > > @@ -3044,6 +3048,11 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> > >  	/* Try to transmit */
> > >  	err = xmit_skb(sq, skb, !use_napi);
> > >  
> > > +	if (use_napi) {
> > > +		virtqueue_set_tx_newbuf_sent(sq->vq, true);
> > > +		virtqueue_set_tx_oldbuf_cleaned(sq->vq, false);
> > > +	}
> > > +
> > >  	/* This should not happen! */
> > >  	if (unlikely(err)) {
> > >  		DEV_STATS_INC(dev, tx_fifo_errors);
> > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > index be7309b1e860..fb2afc716371 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -180,6 +180,11 @@ struct vring_virtqueue {
> > >  	 */
> > >  	bool do_unmap;
> > >  
> > > +	/* Has any new data been sent? */
> > > +	bool is_tx_newbuf_sent;
> > > +	/* Is the old data recently sent cleaned up? */
> > > +	bool is_tx_oldbuf_cleaned;
> > > +
> > >  	/* Head of free buffer list. */
> > >  	unsigned int free_head;
> > >  	/* Number we've added since last sync. */
> > > @@ -2092,6 +2097,9 @@ static struct virtqueue *vring_create_virtqueue_packed(
> > >  	vq->use_dma_api = vring_use_dma_api(vdev);
> > >  	vq->premapped = false;
> > >  	vq->do_unmap = vq->use_dma_api;
> > > +	vq->is_tx_newbuf_sent = false; /* Initially, no new buffer to send. */
> > > +	vq->is_tx_oldbuf_cleaned = true; /* Initially, no old buffer to clean. */
> > > +
> > >  
> > >  	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
> > >  		!context;
> > > @@ -2375,6 +2383,38 @@ bool virtqueue_notify(struct virtqueue *_vq)
> > >  }
> > >  EXPORT_SYMBOL_GPL(virtqueue_notify);
> > >  
> > > +/**
> > > + * virtqueue_set_tx_newbuf_sent - set whether there is new tx buf to send.
> > > + * @_vq: the struct virtqueue
> > > + *
> > > + * If is_tx_newbuf_sent and is_tx_oldbuf_cleaned are both true, the
> > > + * spurious interrupt is caused by polling TX vq in other paths outside
> > > + * the tx irq callback.
> > > + */
> > > +void virtqueue_set_tx_newbuf_sent(struct virtqueue *_vq, bool val)
> > > +{
> > > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > > +
> > > +	vq->is_tx_newbuf_sent = val;
> > > +}
> > > +EXPORT_SYMBOL_GPL(virtqueue_set_tx_newbuf_sent);
> > > +
> > > +/**
> > > + * virtqueue_set_tx_oldbuf_cleaned - set whether there is old tx buf to clean.
> > > + * @_vq: the struct virtqueue
> > > + *
> > > + * If is_tx_oldbuf_cleaned and is_tx_newbuf_sent are both true, the
> > > + * spurious interrupt is caused by polling TX vq in other paths outside
> > > + * the tx irq callback.
> > > + */
> > > +void virtqueue_set_tx_oldbuf_cleaned(struct virtqueue *_vq, bool val)
> > > +{
> > > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > > +
> > > +	vq->is_tx_oldbuf_cleaned = val;
> > > +}
> > > +EXPORT_SYMBOL_GPL(virtqueue_set_tx_oldbuf_cleaned);
> > > +
> > >  /**
> > >   * virtqueue_kick - update after add_buf
> > >   * @vq: the struct virtqueue
> > > @@ -2572,6 +2612,16 @@ irqreturn_t vring_interrupt(int irq, void *_vq)
> > >  	struct vring_virtqueue *vq = to_vvq(_vq);
> > >  
> > >  	if (!more_used(vq)) {
> > > +		/* When the delayed TX interrupt arrives, the old buffers are
> > > +		 * cleaned in other cases(start_xmit and virtnet_poll_cleantx).
> > > +		 * We'd better not identify it as a spurious interrupt,
> > > +		 * otherwise note_interrupt may kill the interrupt.
> > > +		 */
> > > +		if (unlikely(vq->is_tx_newbuf_sent && vq->is_tx_oldbuf_cleaned)) {
> > > +			vq->is_tx_newbuf_sent = false;
> > > +			return IRQ_HANDLED;
> > > +		}
> > > +

I am not merging anything this ugly.

> > >  		pr_debug("virtqueue interrupt with no work for %p\n", vq);
> > >  		return IRQ_NONE;
> > >  	}
> > > @@ -2637,6 +2687,9 @@ static struct virtqueue *__vring_new_virtqueue(unsigned int index,
> > >  	vq->use_dma_api = vring_use_dma_api(vdev);
> > >  	vq->premapped = false;
> > >  	vq->do_unmap = vq->use_dma_api;
> > > +	vq->is_tx_newbuf_sent = false; /* Initially, no new buffer to send. */
> > > +	vq->is_tx_oldbuf_cleaned = true; /* Initially, no old buffer to clean. */
> > > +
> > >  
> > >  	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
> > >  		!context;
> > > diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> > > index ecc5cb7b8c91..ba3be9276c09 100644
> > > --- a/include/linux/virtio.h
> > > +++ b/include/linux/virtio.h
> > > @@ -103,6 +103,9 @@ int virtqueue_resize(struct virtqueue *vq, u32 num,
> > >  int virtqueue_reset(struct virtqueue *vq,
> > >  		    void (*recycle)(struct virtqueue *vq, void *buf));
> > >  
> > > +void virtqueue_set_tx_newbuf_sent(struct virtqueue *vq, bool val);
> > > +void virtqueue_set_tx_oldbuf_cleaned(struct virtqueue *vq, bool val);
> > > +
> > >  struct virtio_admin_cmd {
> > >  	__le16 opcode;
> > >  	__le16 group_type;
> > > -- 
> > > 2.32.0.3.g01195cf9f
> >