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

Jason Wang
2021-05-27T08:41:26Z

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

>
> Thanks,
> Yongji
>