Virtualization - May 2017 - remove function pointer casts and constify function tables

Looks like the culprit is very likely d85b758f72b0 "virtio_net: fix
support for small rings".

After that patch, my NFS server VM stops responding to packets after a
few minutes of testing.  Before that patch, my server keeps working.

--b.

On Tue, May 30, 2017 at 07:26:37PM +0300, Michael S. Tsirkin
wrote:
> On Fri, May 26, 2017 at 03:31:33PM -0400, bfields at fieldses.org wrote:
> > Looks like the culprit is very likely d85b758f72b0 "virtio_net:
fix
> > support for small rings".
> > 
> > After that patch, my NFS server VM stops responding to packets after a
> > few minutes of testing.  Before that patch, my server keeps working.
> > 
> > --b.
> 
> Others complained about that too.
> I'm still trying to reproduce though.
> 
> Meanwhile, could you please locate this line of code:
> +               vi->rq[i].min_buf_len = mergeable_min_buf_len(vi,
vi->rq[i].vq);
> 
> and add something like
>         printk(KERN_ERR, "min buf = 0x%x expected 0x%x size 0x%x big
%d\n",
>                vi->rq[i].min_buf_len, GOOD_PACKET_LEN,
>                virtqueue_get_vring_size(vi->rq[i].vq),
>                (int)vi->big_packets);
> 
> after it?
> Then boot and capture the output.
> 
> Thanks!
> 
Also, can you pls print the mergeable_rx_buffer_size attribute from
sysfs for this device?

> -- 
> MST

On Tue, May 30, 2017 at 08:03:12PM +0300, Michael S. Tsirkin
wrote:
> On Fri, May 26, 2017 at 03:31:33PM -0400, bfields at fieldses.org wrote:
> > Looks like the culprit is very likely d85b758f72b0 "virtio_net:
fix
> > support for small rings".
> > 
> > After that patch, my NFS server VM stops responding to packets after a
> > few minutes of testing.  Before that patch, my server keeps working.
> > 
> > --b.
> 
> 
> So I think I know what caused this: looks like some hypervisors
> aren't prepared to deal with a situation where packet size
> becomes very small.
> 
> But which hypervisors exactly? I'd like to know in order to detect
these
> and decide whether I blacklist bad ones or whitelist known-good ones.
I'm running this under KVM on a Fedora 25 host
(4.10.15-200.fc25.x86_64).  Let me know if any more details about the
setup would be useful.

--b.

On Tue, May 30, 2017 at 07:26:37PM +0300, Michael S. Tsirkin
wrote:
> On Fri, May 26, 2017 at 03:31:33PM -0400, bfields at fieldses.org wrote:
> > Looks like the culprit is very likely d85b758f72b0 "virtio_net:
fix
> > support for small rings".
> > 
> > After that patch, my NFS server VM stops responding to packets after a
> > few minutes of testing.  Before that patch, my server keeps working.
> > 
> > --b.
> 
> Others complained about that too.
> I'm still trying to reproduce though.
> 
> Meanwhile, could you please locate this line of code:
> +               vi->rq[i].min_buf_len = mergeable_min_buf_len(vi,
vi->rq[i].vq);
> 
> and add something like
>         printk(KERN_ERR, "min buf = 0x%x expected 0x%x size 0x%x big
%d\n",
>                vi->rq[i].min_buf_len, GOOD_PACKET_LEN,
>                virtqueue_get_vring_size(vi->rq[i].vq),
>                (int)vi->big_packets);
> 
> after it?
> Then boot and capture the output.
Doesn't look like that code's run on boot; apply the below, boot, and:

 $ dmesg|grep expected

gives no output.

--b.

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 9320d96a1632..b10014f7b480 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2212,6 +2212,10 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
        for (i = 0; i < vi->max_queue_pairs; i++) {
                vi->rq[i].vq = vqs[rxq2vq(i)];
                vi->rq[i].min_buf_len = mergeable_min_buf_len(vi,
vi->rq[i].vq);
+               printk(KERN_ERR, "min buf = 0x%x expected 0x%x size 0x%x
big %d\n",
+                               vi->rq[i].min_buf_len, GOOD_PACKET_LEN,
+                               virtqueue_get_vring_size(vi->rq[i].vq),
+                               (int)vi->big_packets);
                vi->sq[i].vq = vqs[txq2vq(i)];
        }