Re: [PATCH] vhost: fix segfault on bad descriptor address.

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

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?

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.

So maybe we should do virtio reset on port start?

	--yliu