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

[Jason Wang]

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


> Thanks,
> Yongji