Date:   Mon, 31 May 2021 12:27:27 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Jason Wang <jasowang@...hat.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: Re: [PATCH v7 11/12] vduse: Introduce VDUSE - vDPA Device in Userspace

On Fri, May 28, 2021 at 10:31 AM Jason Wang <jasowang@...hat.com> wrote:
>
>
> 在 2021/5/27 下午9:17, Yongji Xie 写道:
> > On Thu, May 27, 2021 at 4:41 PM Jason Wang <jasowang@...hat.com> wrote:
> >>
> >> 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
> > I prefer this way. And I think we also need to do similar things for
> > set/get_vq_state().
>
>
> Yes, I agree.
>

Hi Jason,

Now I'm working on this. But I found the config change notification
must be synchronous in the virtio-blk case, which means the kernel
still needs to wait for the response from userspace in set_config().
Otherwise, some I/Os might still run the old way after we change the
cache_type in sysfs.

The simple ways to solve this problem are:

1. Only support read-only config space, disable WCE as you suggested
2. Add a return value to set_config() and handle the failure only in
virtio-blk driver
3. Print some warnings after timeout since it only affects the
dataplane which is under userspace's control

Any suggestions?

Thanks,
Yongji

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