Re: [PATCH RFC v2 08/12] lib/librte_vhost: vhost-user support

[Tetsuya Mukawa <mukawa-AlSX/UN32fvPDbFq/vQRIQ@public.gmane.org>]

(2014/12/17 12:31), Tetsuya Mukawa wrote:
> (2014/12/17 10:06), Xie, Huawei wrote:
>>>> +{
>>>> +	struct virtio_net *dev = get_device(ctx);
>>>> +
>>>> +	/* We have to stop the queue (virtio) if it is running. */
>>>> +	if (dev->flags & VIRTIO_DEV_RUNNING)
>>>> +		notify_ops->destroy_device(dev);
>>> I have an one concern about finalization of vrings.
>>> Can vhost-backend stop accessing RX/TX to the vring before replying to
>>> this message?
>>>
>>> QEMU sends this message when virtio-net device is finalized by
>>> virtio-net driver on the guest.
>>> After finalization, memories used by the vring will be freed by
>>> virtio-net driver, because these memories are allocated by virtio-net
>>> driver.
>>> Because of this, I guess vhost-backend must stop accessing to vring
>>> before replying to this message.
>>>
>>> I am not sure what is a good way to stop accessing.
>>> One idea is adding a condition checking when rte_vhost_dequeue_burst()
>>> and rte_vhost_enqueue_burst() is called.
>>> Anyway we probably need to wait for stopping access before replying.
>>>
>>> Thanks,
>>> Tetsuya
>>>
>> I think we have discussed the similar question.
> Sorry, probably I might not be able to got your point correctly at the
> former email.
>
>> It is actually the same issue whether the virtio APP in guest is crashed, or is finalized.
> I guess when the APP is finalized correctly, we can have a solution.
> Could you please read comment I wrote later?
>
>> The virtio APP will only write to the STATUS register without waiting/syncing to vhost backend.
> Yes, virtio APP only write to the STATUS register. I agree with it.
>
> When the register is written by guest, KVM will catch it, and the
> context will be change to QEMU. And QEMU will works like below.
> (Also while doing following steps, guest is waiting because the context
> is in QEMU)
>
> Could you please see below with latest QEMU code?
> 1. virtio_ioport_write() [hw/virtio/virtio-pci.c] <= virtio APP will
> wait for replying of this function.
> 2. virtio_set_status() [hw/virtio/virtio.c]
> 3. virtio_net_set_status() [hw/net/virtio-net.c]
> 4. virtio_net_vhost_status() [hw/net/virtio-net.c]
> 5. vhost_net_stop() [hw/net/vhost_net.c]
> 6. vhost_net_stop_one() [hw/net/vhost_net.c]
> 7. vhost_dev_stop() [hw/virtio/vhost.c]
> 8. vhost_virtqueue_stop() [hw/virtio/vhost.c]
> 9. vhost_user_call() [virtio/vhost-user.c]
> 10. VHOST_USER_GET_VRING_BASE message is sent to backend. And waiting
> for backend reply.
>
> When the vhost-user backend receives GET_VRING_BASE, I guess the guest
> APP is stopped.
> Also QEMU will wait for vhost-user backend reply because GET_VRING_BASE
> is synchronous message.
> Because of above, I guess virtio APP can wait for vhost-backend
> finalization.
>
>> After that, not only the guest memory pointed by vring entry but also the vring itself isn't usable any more.
>> The memory for vring or pointed by vring entry might be used by other APPs.
> I agree.
>
>> This will crash guest(rather than the vhost, do you agree?).
> I guess we cannot assume how the freed memory is used.
> In some cases, a new APP still works, but vhost backend can access
> inconsistency vring structure.
> In the case vhost backend could receive illegal packets.
> For example, avail_idx value might be changed to be 0xFFFF by a new APP.
> (I am not sure RX/TX functions can handle such a value correctly)
>
> Anyway, my point is if we can finalize vhost backend correctly, we only
> need to take care of crashing case.
> (If so, it's very nice :))
> So let's make sure whether virtio APP can wait for finalization, or not.
> I am thinking how to do it now.
>

I added sleep() like below.

--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -300,7 +300,10 @@ static void virtio_ioport_write(void *opaque,
uint32_t addr, uint32_t val)
             virtio_pci_stop_ioeventfd(proxy);
         }
 
         virtio_set_status(vdev, val & 0xFF);
+        if (val == 0)
+            sleep(10);
 
         if (val & VIRTIO_CONFIG_S_DRIVER_OK) {
             virtio_pci_start_ioeventfd(proxy);

When I type 'dpdk_nic_bind.py' to cause GET_VRING_BASE, this command
takes 10 seconds to be finished.
So we can assume that virtio APP is able to wait for finalization of
vhost backend.

Thanks,
Tetsuya

>> If you mean this issue, I think we have no solution but one walk around: keep the huge page files of crashed app, and 
>> bind virtio to igb_uio and then delete the huge page files.
> Yes I agree.
> If the virtio APP is crashed, this will be a solution.
>
> Thanks,
> Tetsuya
>
>> In our implementation, when vhost sends the message,  we will call the destroy_device provided by the vSwitch to ask the
>> vSwitch to stop processing the vring, but this willn't solve the issue I mention above, because the virtio APP in guest will n't 
>> wait us.
>>
>> Could you explain a bit more? Is it the same issue?
>>
>>
>> -huawei
>>
>>
>>
>