Date:   Fri, 28 May 2021 09:33:43 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Yongji Xie <xieyongji@...edance.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Parav Pandit <parav@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christian Brauner <christian.brauner@...onical.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, bcrl@...ck.org,
        Jonathan Corbet <corbet@....net>,
        Mika Penttilä <mika.penttila@...tfour.com>,
        Dan Carpenter <dan.carpenter@...cle.com>, joro@...tes.org,
        virtualization <virtualization@...ts.linux-foundation.org>,
        netdev@...r.kernel.org, kvm <kvm@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 11/12] vduse: Introduce VDUSE - vDPA Device in
 Userspace


在 2021/5/27 下午6:14, Yongji Xie 写道:
> On Thu, May 27, 2021 at 4:43 PM Jason Wang <jasowang@...hat.com> wrote:
>>
>> 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@...hat.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@...hat.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@...hat.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.
>>
> Do you mean terminate the data path in vdpa_reset().


Yes.


>   Is it ok to just
> notify userspace to stop data path asynchronously?


For well-behaved userspace, yes but no for buggy or malicious ones.

I had an idea, how about terminate IOTLB in this case? Then we're in 
fact turn datapath off.

Thanks


>   Userspace should
> not be able to do any I/O at that time because the iotlb mapping is
> already removed.
>
> Thanks,
> Yongji
>

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