Date:   Fri, 28 May 2021 14:38:54 +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 2021/5/28 上午11:54, Yongji Xie 写道:
> On Fri, May 28, 2021 at 9:33 AM Jason Wang <jasowang@...hat.com> wrote:
>>
>> 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.
>>
> But the buggy or malicious daemons can't do anything if my
> understanding is correct.


You're right. I originally thought there can still have bouncing. But 
consider we don't do that during fault.

It should be safe.


>
>> I had an idea, how about terminate IOTLB in this case? Then we're in
>> fact turn datapath off.
>>
> Sorry, I didn't get your point here. What do you mean by terminating
> IOTLB?


I meant terminate the bouncing but it looks safe after a second thought :)

Thanks


>   Remove iotlb mapping? But userspace can still access the mapped
> region.
>
> Thanks,
> Yongji
>

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