Re: [PATCH v2] virtio: fix segfault when transmit pkts

["Tan, Jianfeng" <jianfeng.tan@intel.com>]

Hi Yuanhan,

> -----Original Message-----
> From: Yuanhan Liu [mailto:yuanhan.liu@linux.intel.com]
> Sent: Tuesday, April 26, 2016 11:43 AM
> To: Tan, Jianfeng
> Cc: dev@dpdk.org; Xie, Huawei
> Subject: Re: [PATCH v2] virtio: fix segfault when transmit pkts
> 
> On Mon, Apr 25, 2016 at 02:37:45AM +0000, Jianfeng Tan wrote:
> > Issue: when using virtio nic to transmit pkts, it causes segment fault.
> >
> > How to reproduce:
> > Basically, we need to construct a case with vm send packets to vhost-user,
> > and this issue does not happen when transmitting packets using indirect
> > desc. Besides, make sure all descriptors are exhausted before vhost
> > dequeues any packets.
> >
> > 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
> >
> > b. start a qemu with a virtio nic connected with the vhost-user port, just
> > make sure mrg_rxbuf is enabled.
> >
> > c. enable testpmd on the host.
> >   testpmd> set fwd io
> >   testpmd> start (better without start vhost-user)
> >
> > 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 (), not necessary to update
> after
> > the loop.
> 
> That's not a right "because": you were stating a fact of the right way
> to do setup desc flags, but not the cause of this bug.
> 
> > (And if we do that after the loop, if all descs could have run out,
> > idx is VQ_RING_DESC_CHAIN_END (32768), use this idx to reference the
> start_dp
> > array will lead to segment fault.)
> 
> And that's the cause. So, you should state the cause first, then the fix
> (which we already have), but not in the verse order you just did.
> 
> So, I'd like to reword the commit log a bit, to something like following.
> What do you think of it? If no objection, I could merge it soon. Thanks
> for the fix, BTW!
> 

Your refinement sounds much better, thanks!

Jianfeng