[dpdk-dev] [PATCH] virtio: fix segfault when transmit pkts

[Yuanhan Liu]

On Thu, Apr 21, 2016 at 12:36:10PM +0000, Jianfeng Tan wrote:
> Issue: when using virtio nic to transmit pkts, it causes segment fault.

Jianfeng,

It's good to describe the issues, steps to reproduce it and how to fix
it. But you don't have to tell it like filling a form. Instead, you
could try to describe in plain English.

> How to reproduce:
> a. start testpmd with vhost.
> $testpmd -c 0x3 -n 4 --socket-mem 1024,0 --no-pci \
>   --vdev 'eth_vhost0,iface=/tmp/sock0,queues=1' -- -i --nb-cores=1

I personally would suggest to add some indentations (and some whitespace
lines), like

a. start testpmd with vhost.
   $ testpmd -c 0x3 -n 4 ... \
     --vdev '...' ...

b. something else ...
   some more explanations.

And, do not quote a command line like "$testpmd ...", it's like a var
in base in this way. Instead, add a space after "$ ", like what I did
above.

> b. start a qemu with a virtio nic connected with the vhost-user port.

This should be enough. I didn't find any special options below,
therefore, above words is enough. However, if it's some specific
option introduces a bug, you could claim it aloud then.

> $qemu -smp cores=2,sockets=1 -cpu host -enable-kvm vm-0.img -vnc :5 -m 4G \
>   -object memory-backend-file,id=mem,size=4096M,mem-path=<path>,share=on \
>   -numa node,memdev=mem -mem-prealloc \
>   -chardev socket,id=char1,path=$sock_vhost \
>   -netdev type=vhost-user,id=net1,chardev=char1 \
>   -device virtio-net-pci,netdev=net1,mac=00:01:02:03:04:05
> c. enable testpmd on the host.
> testpmd> set fwd io
> testpmd> start
> d. start testpmd in VM.
> $testpmd -c 0x3 -n 4 -m 1024 -- -i --disable-hw-vlan-filter --txqflags=0xf01
> testpmd> set fwd txonly
> testpmd> start
> 
> How to fix: this bug is because inside virtqueue_enqueue_xmit(), the flag of
> desc has been updated inside the do {} while (); and after the loop, all descs
> could have run out, so idx is VQ_RING_DESC_CHAIN_END (32768), use this idx to
> reference the start_dp array will lead to segment fault.

You missed a fix line here.

> Signed-off-by: Jianfeng Tan <jianfeng.tan at intel.com>
> ---
>  drivers/net/virtio/virtio_rxtx.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
> index ef21d8e..432aeab 100644
> --- a/drivers/net/virtio/virtio_rxtx.c
> +++ b/drivers/net/virtio/virtio_rxtx.c
> @@ -271,8 +271,6 @@ virtqueue_enqueue_xmit(struct virtqueue *txvq, struct rte_mbuf *cookie,
>  		idx = start_dp[idx].next;
>  	} while ((cookie = cookie->next) != NULL);
>  
> -	start_dp[idx].flags &= ~VRING_DESC_F_NEXT;

This looks a good fix to me. I'm just wondering why we never met it
before, and on which specific case do we meet it? Talking about that,
seems like "set fwd txonly" is with high suspicious.

	--yliu