Re: [PATCH v7 11/12] vduse: Introduce VDUSE - vDPA Device in Userspace

[Jason Wang <jasowang@redhat.com>]

在 2021/5/27 下午4:41, Jason Wang 写道:
>
> 在 2021/5/27 下午3:34, Yongji Xie 写道:
>> On Thu, May 27, 2021 at 1:40 PM Jason Wang <jasowang@redhat.com> wrote:
>>>
>>> 在 2021/5/27 下午1:08, Yongji Xie 写道:
>>>> On Thu, May 27, 2021 at 1:00 PM Jason Wang <jasowang@redhat.com> 
>>>> wrote:
>>>>> 在 2021/5/27 下午12:57, Yongji Xie 写道:
>>>>>> On Thu, May 27, 2021 at 12:13 PM Jason Wang <jasowang@redhat.com> 
>>>>>> wrote:
>>>>>>> 在 2021/5/17 下午5:55, Xie Yongji 写道:
>>>>>>>> +
>>>>>>>> +static int vduse_dev_msg_sync(struct vduse_dev *dev,
>>>>>>>> +                           struct vduse_dev_msg *msg)
>>>>>>>> +{
>>>>>>>> +     init_waitqueue_head(&msg->waitq);
>>>>>>>> +     spin_lock(&dev->msg_lock);
>>>>>>>> +     vduse_enqueue_msg(&dev->send_list, msg);
>>>>>>>> +     wake_up(&dev->waitq);
>>>>>>>> +     spin_unlock(&dev->msg_lock);
>>>>>>>> +     wait_event_killable(msg->waitq, msg->completed);
>>>>>>> What happens if the userspace(malicous) doesn't give a response 
>>>>>>> forever?
>>>>>>>
>>>>>>> It looks like a DOS. If yes, we need to consider a way to fix that.
>>>>>>>
>>>>>> How about using wait_event_killable_timeout() instead?
>>>>> Probably, and then we need choose a suitable timeout and more 
>>>>> important,
>>>>> need to report the failure to virtio.
>>>>>
>>>> Makes sense to me. But it looks like some
>>>> vdpa_config_ops/virtio_config_ops such as set_status() didn't have a
>>>> return value.  Now I add a WARN_ON() for the failure. Do you mean we
>>>> need to add some change for virtio core to handle the failure?
>>>
>>> Maybe, but I'm not sure how hard we can do that.
>>>
>> We need to change all virtio device drivers in this way.
>
>
> Probably.
>
>
>>
>>> We had NEEDS_RESET but it looks we don't implement it.
>>>
>> Could it handle the failure of get_feature() and get/set_config()?
>
>
> Looks not:
>
> "
>
> The device SHOULD set DEVICE_NEEDS_RESET when it enters an error state 
> that a reset is needed. If DRIVER_OK is set, after it sets 
> DEVICE_NEEDS_RESET, the device MUST send a device configuration change 
> notification to the driver.
>
> "
>
> This looks implies that NEEDS_RESET may only work after device is 
> probed. But in the current design, even the reset() is not reliable.
>
>
>>
>>> Or a rough idea is that maybe need some relaxing to be coupled loosely
>>> with userspace. E.g the device (control path) is implemented in the
>>> kernel but the datapath is implemented in the userspace like TUN/TAP.
>>>
>> I think it can work for most cases. One problem is that the set_config
>> might change the behavior of the data path at runtime, e.g.
>> virtnet_set_mac_address() in the virtio-net driver and
>> cache_type_store() in the virtio-blk driver. Not sure if this path is
>> able to return before the datapath is aware of this change.
>
>
> Good point.
>
> But set_config() should be rare:
>
> E.g in the case of virtio-net with VERSION_1, config space is read 
> only, and it was set via control vq.
>
> For block, we can
>
> 1) start from without WCE or
> 2) we add a config change notification to userspace or
> 3) extend the spec to use vq instead of config space
>
> Thanks


Another thing if we want to go this way:

We need find a way to terminate the data path from the kernel side, to 
implement to reset semantic.

Thanks


>
>
>>
>> Thanks,
>> Yongji
>>

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