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

[Jason Wang]

在 2021/5/28 上午11:54, Yongji Xie 写道:
> On Fri, May 28, 2021 at 9:33 AM Jason Wang <jasowang@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > > > > > 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@xxxxxxxxxx>
> > > > > > > > > > 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