linux-kernel - Re: [PATCH v7 11/12] vduse: Introduce VDUSE

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Message-ID: <00ded99f-91b6-ba92-5d92-2366b163f129@redhat.com>
Date:   Thu, 27 May 2021 16:41:01 +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


>
> Thanks,
> Yongji
>