[dpdk-dev] [PATCH] vhost: fix segfault on bad descriptor address.

Yuanhan Liu yuanhan.liu at linux.intel.com
Tue Jul 12 04:43:05 CEST 2016


On Mon, Jul 11, 2016 at 02:47:56PM +0300, Ilya Maximets wrote:
> On 11.07.2016 14:05, Yuanhan Liu wrote:
> > On Mon, Jul 11, 2016 at 12:50:24PM +0300, Ilya Maximets wrote:
> >> On 11.07.2016 11:38, Yuanhan Liu wrote:
> >>> On Sun, Jul 10, 2016 at 09:17:31PM +0800, Yuanhan Liu wrote:
> >>>> On Fri, Jul 08, 2016 at 02:48:56PM +0300, Ilya Maximets wrote:
> >>>>>
> >>>>> Another point is that crash constantly happens on queue_id=3 (second RX queue) in
> >>>>> my scenario. It is newly allocated virtqueue while reconfiguration from rxq=1 to
> >>>>> rxq=2.
> >>>>
> >>>> That's a valuable message: what's your DPDK HEAD commit while triggering
> >>>> this issue?
> >>
> >> fbfd99551ca3 ("mbuf: add raw allocation function")
> >>
> >>>
> >>> I guess I have understood what goes wrong in you case.
> >>>
> >>> I would guess that your vhost has 2 queues (here I mean queue-pairs,
> >>> including one Tx and Rx queue; below usage is the same) configured,
> >>> so does to your QEMU. However, you just enabled 1 queue while starting
> >>> testpmd inside the guest, and you want to enable 2 queues by running
> >>> following testpmd commands:
> >>>
> >>>     stop
> >>>     port stop all
> >>>     port config all rxq 2
> >>>     port config all txq 2
> >>>     port start all
> >>>
> >>> Badly, that won't work for current virtio PMD implementation, and what's
> >>> worse, it triggers a vhost crash, the one you saw.
> >>>
> >>> Here is how it comes. Since you just enabled 1 queue while starting
> >>> testpmd, it will setup 1 queue only, meaning only one queue's **valid**
> >>> information will be sent to vhost. You might see SET_VRING_ADDR
> >>> (and related vhost messages) for the other queue as well, but they
> >>> are just the dummy messages: they don't include any valid/real
> >>> information about the 2nd queue: the driver don't setup it after all.
> >>>
> >>> So far, so good. It became broken when you run above commands. Those
> >>> commands do setup for the 2nd queue, however, they failed to trigger
> >>> the QEMU virtio device to start the vhost-user negotiation, meaning
> >>> no SET_VRING_ADDR will be sent for the 2nd queue, leaving vhost
> >>> untold and not updated.
> >>>
> >>> What's worse, above commands trigger the QEMU to send SET_VRING_ENABLE
> >>> messages, to enable all the vrings. And since the vrings for the 2nd
> >>> queue are not properly configured, the crash happens.
> >>
> >> Hmm, why 2nd queue works properly with my fix to vhost in this case?
> > 
> > Hmm, really? You are sure that data flows in your 2nd queue after those
> > commands? From what I know is that your patch just avoid a crash, but
> > does not fix it.
> 
> Oh, sorry. Yes, it doesn't work. With my patch applied I have a QEMU hang.

The crash actually could be avoided by commit 0823c1cb0a73 ("vhost:
workaround stale vring base"), accidentally. That's why I asked you
above the HEAD commit you were using.

> >>> So maybe we should do virtio reset on port start?
> >>
> >> I guess it was removed by this patch:
> >> a85786dc816f ("virtio: fix states handling during initialization").
> > 
> > Seems yes.

Actually, we should not do that: do reset on port start. The right fix
should be allocating MAX queues virtio device supports (2 here). This
would allow us changing the queue number dynamically.

But this doesn't sound a simple fix; it involves many code changes, due
to it was not designed this way before. Therefore, we will not fix it
in this release, due to it's too late. Let's fix it in the next release
instead. For the crash issue, it will not happen with the latest HEAD.
Though it's accident fix, I think we are fine here.

	--yliu


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