[dpdk-dev] virtio kills qemu VM after stopping/starting ports

[Kyle Larose]

> -----Original Message-----
> From: Yuanhan Liu [mailto:yuanhan.liu at linux.intel.com]
> Sent: Monday, September 05, 2016 12:10 AM
> To: Kyle Larose
> Cc: dev at dpdk.org; huawei.xie at intel.com; jianfeng.tan at intel.com
> Subject: Re: virtio kills qemu VM after stopping/starting ports
> 
> On Thu, Sep 01, 2016 at 08:53:31PM +0000, Kyle Larose wrote:
> > Hello everyone,
> 
> Hi,
> 
> Firstly, thanks for the report and detailed analysis!
> 
> >
> > In my own testing, I recently stumbled across an issue where I could get qemu
> to exit when sending traffic to my application. To do this, I simply needed to do
> the following:
> >
> > 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
> >
> > At this point, I would lose connectivity to my VM.  Further investigation
> revealed qemu exiting with the following log:
> >
> > 	2016-09-01T15:45:32.119059Z qemu-kvm: Guest moved used index
> from 5
> > to 1
> >
> > I found the following bug report against qemu, reported by a user of
> > DPDK: https://bugs.launchpad.net/qemu/+bug/1558175
> >
> > That thread seems to have stalled out, so I think we probably should deal with
> the problem within DPDK itself. Either way, later in the bug report chain, we
> see a link to this patch to DPDK:
> http://dpdk.org/browse/dpdk/commit/?id=9a0615af774648. The submitter of
> the bug report claims that this patch fixes the problem. Perhaps it does.
> However, it introduces a new problem: If I remove the patch, I cannot
> reproduce the problem. So, that leads me to believe that it has caused a
> regression.
> 
> Yes, it is a regression from that point of view.
> 
> > To summarize the patch’s changes, it basically changes the virtio_dev_stop
> function to flag the device as stopped, and stops the device when
> closing/uninitializing it. However, there is a seemingly unintended side-effect.
> >
> > In virtio_dev_start, we have the following block of code:
> >
> > 	/* On restart after stop do not touch queues */
> > 	if (hw->started)
> > 		return 0;
> >
> > 	/* Do final configuration before rx/tx engine starts */
> > 	virtio_dev_rxtx_start(dev);
> >
> > ….
> >
> > 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.
> >
> > I made a change locally to break the state of the device into two: started and
> opened. The devices starts out neither started nor opened. If the device is
> accepting packets, it is started. If the device has set up its queues, it is opened.
> Stopping the device does not close the device. This allows me to change the
> check above to:
> >
> >  	if (hw->opened) {
> > 		hw->started=1
> > 		return 0;
> > 	}
> 
> It would work in your case, but it makes thing complex.
> 
> So, I talked with Jianfeng and revisited the original issue he meant to
> fix: failure (maybe crash) on stop, re-configure queue number and restart.
> 
> Yes, that case is broken, but the fix wasn't right, neither: we can't simply re-
> alloc and re-setup queue on start, because vhost is only aware of the first setup.
> You could check following link for more information, including the right fix (you
> need follow the discussion to find that).
> 
> In summary, I will revert commit 9a0615af774 (and carry it to the stable
> branch as well). Later, I will fix the virtio multiple queue issue.
> 

Alright, so we should probably reject my patch, then. :) http://dpdk.org/dev/patchwork/patch/15596/



> 	--yliu

Thanks for getting back to me on this.

Kyle