Subject: Re: [PATCH] vhost: break out of polling loop on error

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 28 Jun 2010 20:25:59 +0300

On Mon, Jun 28, 2010 at 10:11:43AM -0700, Sridhar Samudrala wrote:
> On Sun, 2010-06-27 at 11:59 +0300, Michael S. Tsirkin wrote:
> > When ring parsing fails, we currently handle this
> > as ring empty condition. This means that we enable
> > kicks and recheck ring empty: if this not empty,
> > we re-start polling which of course will fail again.
> > 
> > Instead, let's return a negative error code and stop polling.
> 
> One minor comment on error return below. With that change,
> 
> Acked-by: Sridhar Samudrala <[email-masked]>

Right. In fact, we don't really use the return code,
and the generated binary is smaller if we return
-EINVAL always.

So that's what I'll do.

> > 
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > ---
> > 
> > Dave, I'm sending this out so it can get reviewed.
> > I'll put this on my vhost tree
> > so no need for you to pick this patch directly.
> > 
> >  drivers/vhost/net.c   |   12 ++++++++++--
> >  drivers/vhost/vhost.c |   33 +++++++++++++++++----------------
> >  drivers/vhost/vhost.h |    8 ++++----
> >  3 files changed, 31 insertions(+), 22 deletions(-)
> > 
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index 0f41c91..54096ee 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -98,7 +98,8 @@ static void tx_poll_start(struct vhost_net *net, struct socket *sock)
> >  static void handle_tx(struct vhost_net *net)
> >  {
> >  	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> > -	unsigned head, out, in, s;
> > +	unsigned out, in, s;
> > +	int head;
> >  	struct msghdr msg = {
> >  		.msg_name = NULL,
> >  		.msg_namelen = 0,
> > @@ -135,6 +136,9 @@ static void handle_tx(struct vhost_net *net)
> >  					 ARRAY_SIZE(vq->iov),
> >  					 &out, &in,
> >  					 NULL, NULL);
> > +		/* On error, stop handling until the next kick. */
> > +		if (head < 0)
> > +			break;
> >  		/* Nothing new?  Wait for eventfd to tell us they refilled. */
> >  		if (head == vq->num) {
> >  			wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> > @@ -192,7 +196,8 @@ static void handle_tx(struct vhost_net *net)
> >  static void handle_rx(struct vhost_net *net)
> >  {
> >  	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_RX];
> > -	unsigned head, out, in, log, s;
> > +	unsigned out, in, log, s;
> > +	int head;
> >  	struct vhost_log *vq_log;
> >  	struct msghdr msg = {
> >  		.msg_name = NULL,
> > @@ -228,6 +233,9 @@ static void handle_rx(struct vhost_net *net)
> >  					 ARRAY_SIZE(vq->iov),
> >  					 &out, &in,
> >  					 vq_log, &log);
> > +		/* On error, stop handling until the next kick. */
> > +		if (head < 0)
> > +			break;
> >  		/* OK, now we need to know about added descriptors. */
> >  		if (head == vq->num) {
> >  			if (unlikely(vhost_enable_notify(vq))) {
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 3b83382..5ccd384 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -873,12 +873,13 @@ static unsigned get_indirect(struct vhost_dev *dev, struct vhost_virtqueue *vq,
> >   * number of output then some number of input descriptors, it's actually two
> >   * iovecs, but we pack them into one and note how many of each there were.
> >   *
> > - * This function returns the descriptor number found, or vq->num (which
> > - * is never a valid descriptor number) if none was found. */
> > -unsigned vhost_get_vq_desc(struct vhost_dev *dev, struct vhost_virtqueue *vq,
> > -			   struct iovec iov[], unsigned int iov_size,
> > -			   unsigned int *out_num, unsigned int *in_num,
> > -			   struct vhost_log *log, unsigned int *log_num)
> > + * This function returns the descriptor number found, or vq->num (which is
> > + * never a valid descriptor number) if none was found.  A negative code is
> > + * returned on error. */
> > +int vhost_get_vq_desc(struct vhost_dev *dev, struct vhost_virtqueue *vq,
> > +		      struct iovec iov[], unsigned int iov_size,
> > +		      unsigned int *out_num, unsigned int *in_num,
> > +		      struct vhost_log *log, unsigned int *log_num)
> >  {
> >  	struct vring_desc desc;
> >  	unsigned int i, head, found = 0;
> > @@ -890,13 +891,13 @@ unsigned vhost_get_vq_desc(struct vhost_dev *dev, struct vhost_virtqueue *vq,
> >  	if (get_user(vq->avail_idx, &vq->avail->idx)) {
> >  		vq_err(vq, "Failed to access avail idx at %p\n",
> >  		       &vq->avail->idx);
> > -		return vq->num;
> > +		return -EFAULT;
> >  	}
> > 
> >  	if ((u16)(vq->avail_idx - last_avail_idx) > vq->num) {
> >  		vq_err(vq, "Guest moved used index from %u to %u",
> >  		       last_avail_idx, vq->avail_idx);
> > -		return vq->num;
> > +		return -EFAULT;
> 
> This should be -EINVAL
> >  	}
> > 
> >  	/* If there's nothing new since last we looked, return invalid. */
> > @@ -912,14 +913,14 @@ unsigned vhost_get_vq_desc(struct vhost_dev *dev, struct vhost_virtqueue *vq,
> >  		vq_err(vq, "Failed to read head: idx %d address %p\n",
> >  		       last_avail_idx,
> >  		       &vq->avail->ring[last_avail_idx % vq->num]);
> > -		return vq->num;
> > +		return -EFAULT;
> >  	}
> > 
> >  	/* If their number is silly, that's an error. */
> >  	if (head >= vq->num) {
> >  		vq_err(vq, "Guest says index %u > %u is available",
> >  		       head, vq->num);
> > -		return vq->num;
> > +		return -EINVAL;
> >  	}
> > 
> >  	/* When we start there are none of either input nor output. */
> > @@ -933,19 +934,19 @@ unsigned vhost_get_vq_desc(struct vhost_dev *dev, struct vhost_virtqueue *vq,
> >  		if (i >= vq->num) {
> >  			vq_err(vq, "Desc index is %u > %u, head = %u",
> >  			       i, vq->num, head);
> > -			return vq->num;
> > +			return -EINVAL;
> >  		}
> >  		if (++found > vq->num) {
> >  			vq_err(vq, "Loop detected: last one at %u "
> >  			       "vq size %u head %u\n",
> >  			       i, vq->num, head);
> > -			return vq->num;
> > +			return -EINVAL;
> >  		}
> >  		ret = copy_from_user(&desc, vq->desc + i, sizeof desc);
> >  		if (ret) {
> >  			vq_err(vq, "Failed to get descriptor: idx %d addr %p\n",
> >  			       i, vq->desc + i);
> > -			return vq->num;
> > +			return -EFAULT;
> >  		}
> >  		if (desc.flags & VRING_DESC_F_INDIRECT) {
> >  			ret = get_indirect(dev, vq, iov, iov_size,
> > @@ -954,7 +955,7 @@ unsigned vhost_get_vq_desc(struct vhost_dev *dev, struct vhost_virtqueue *vq,
> >  			if (ret < 0) {
> >  				vq_err(vq, "Failure detected "
> >  				       "in indirect descriptor at idx %d\n", i);
> > -				return vq->num;
> > +				return ret;
> >  			}
> >  			continue;
> >  		}
> > @@ -964,7 +965,7 @@ unsigned vhost_get_vq_desc(struct vhost_dev *dev, struct vhost_virtqueue *vq,
> >  		if (ret < 0) {
> >  			vq_err(vq, "Translation failure %d descriptor idx %d\n",
> >  			       ret, i);
> > -			return vq->num;
> > +			return ret;
> >  		}
> >  		if (desc.flags & VRING_DESC_F_WRITE) {
> >  			/* If this is an input descriptor,
> > @@ -981,7 +982,7 @@ unsigned vhost_get_vq_desc(struct vhost_dev *dev, struct vhost_virtqueue *vq,
> >  			if (*in_num) {
> >  				vq_err(vq, "Descriptor has out after in: "
> >  				       "idx %d\n", i);
> > -				return vq->num;
> > +				return -EINVAL;
> >  			}
> >  			*out_num += ret;
> >  		}
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index 44591ba..11ee13d 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -120,10 +120,10 @@ long vhost_dev_ioctl(struct vhost_dev *, unsigned int ioctl, unsigned long arg);
> >  int vhost_vq_access_ok(struct vhost_virtqueue *vq);
> >  int vhost_log_access_ok(struct vhost_dev *);
> > 
> > -unsigned vhost_get_vq_desc(struct vhost_dev *, struct vhost_virtqueue *,
> > -			   struct iovec iov[], unsigned int iov_count,
> > -			   unsigned int *out_num, unsigned int *in_num,
> > -			   struct vhost_log *log, unsigned int *log_num);
> > +int vhost_get_vq_desc(struct vhost_dev *, struct vhost_virtqueue *,
> > +		      struct iovec iov[], unsigned int iov_count,
> > +		      unsigned int *out_num, unsigned int *in_num,
> > +		      struct vhost_log *log, unsigned int *log_num);
> >  void vhost_discard_vq_desc(struct vhost_virtqueue *);
> > 
> >  int vhost_add_used(struct vhost_virtqueue *, unsigned int head, int len);