Subject: Re: [PATCH net v2 2/2] virtio_net: Do not send RSS key if it is not supported

From: Breno Leitao <[email-masked]>
Date: Wed, 27 Mar 2024 06:44:31 -0700

On Wed, Mar 27, 2024 at 10:27:58AM +0800, Heng Qi wrote:
> 
> 
> 在 2024/3/26 下午11:19, Breno Leitao 写道:
> > There is a bug when setting the RSS options in virtio_net that can break
> > the whole machine, getting the kernel into an infinite loop.
> > 
> > Running the following command in any QEMU virtual machine with virtionet
> > will reproduce this problem:
> > 
> >      # ethtool -X eth0  hfunc toeplitz
> > 
> > This is how the problem happens:
> > 
> > 1) ethtool_set_rxfh() calls virtnet_set_rxfh()
> > 
> > 2) virtnet_set_rxfh() calls virtnet_commit_rss_command()
> > 
> > 3) virtnet_commit_rss_command() populates 4 entries for the rss
> > scatter-gather
> > 
> > 4) Since the command above does not have a key, then the last
> > scatter-gatter entry will be zeroed, since rss_key_size == 0.
> > sg_buf_size = vi->rss_key_size;
> > 
> > 5) This buffer is passed to qemu, but qemu is not happy with a buffer
> > with zero length, and do the following in virtqueue_map_desc() (QEMU
> > function):
> > 
> >    if (!sz) {
> >        virtio_error(vdev, "virtio: zero sized buffers are not allowed");
> > 
> > 6) virtio_error() (also QEMU function) set the device as broken
> > 
> >      vdev->broken = true;
> > 
> > 7) Qemu bails out, and do not repond this crazy kernel.
> > 
> > 8) The kernel is waiting for the response to come back (function
> > virtnet_send_command())
> > 
> > 9) The kernel is waiting doing the following :
> > 
> >        while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> > 	     !virtqueue_is_broken(vi->cvq))
> > 	      cpu_relax();
> > 
> > 10) None of the following functions above is true, thus, the kernel
> > loops here forever. Keeping in mind that virtqueue_is_broken() does
> > not look at the qemu `vdev->broken`, so, it never realizes that the
> > vitio is broken at QEMU side.
> > 
> > Fix it by not sending RSS commands if the feature is not available in
> > the device.
> > 
> > Fixes: c7114b1249fa ("drivers/net/virtio_net: Added basic RSS support.")
> > Cc: [email-masked]
> > Cc: [email-masked]
> > Signed-off-by: Breno Leitao <[email-masked]>
> > ---
> >   drivers/net/virtio_net.c | 3 +++
> >   1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index c640fdf28fc5..e6b0eaf08ac2 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -3809,6 +3809,9 @@ static int virtnet_set_rxfh(struct net_device *dev,
> >   	struct virtnet_info *vi = netdev_priv(dev);
> >   	int i;
> > +	if (!vi->has_rss && !vi->has_rss_hash_report)
> > +		return -EOPNOTSUPP;
> > +
> 
> Why not make the second patch as the first, this seems to work better.

Sure, that works for me. Let me update it in v2.