Re: [dpdk-dev] [PATCH 1/8] net/virtio: revert "virtio: fix restart"

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Thu, Nov 03, 2016 at 09:36:52PM +0100, Maxime Coquelin wrote:
> Hi Yuanhan,
> 
> On 11/03/2016 05:09 PM, Yuanhan Liu wrote:
> >This reverts commit 9a0615af7746 ("virtio: fix restart"); conflict is
> >manually addressed.
> >
> >Kyle reported an issue with above commit
> >
> >    qemu-kvm: Guest moved used index from 5 to 1
> >
> >with following steps,
> >
> >    1) Start my virtio interfaces
> >    2) Send some traffic into/out of the interfaces
> >    3) Stop the interfaces
> >    4) Start the interfaces
> >    5) Send some more traffic
> >
> >And here are some quotes from Kyle's analysis,
> >
> >    Prior to the patch, if an interface were stopped then started, without
> >    restarting the application, the queues would be left as-is, because
> >    hw->started would be set to 1. Now, calling stop sets hw->started to 0,
> >    which means the next call to start will "touch the queues". This is the
> >    unintended side-effect that causes the problem.
> 
> Maybe a good idea to explain what is the problem the revert aims to fix.

It aims to fix the issue, by "not touching the queues" on restart.

> It does not seem to be clearly stated in the commit message.

I was thinking the quote from Kyle is enough. How about following supplement:

    We should not touch the queues once the init is done, otherwise, the
    vring state of virtio PMD driver and vhost-user would be inconsistent,
    leading some issue like above.

    Thus this patch is reverted.

Better now?

	--yliu