Re: [PATCH v8 10/10] Documentation: Add documentation for VDUSE

[Stefan Hajnoczi]

On Thu, Jul 08, 2021 at 12:17:56PM +0800, Jason Wang wrote:
> 
> 在 2021/7/7 下午11:54, Stefan Hajnoczi 写道:
> > On Wed, Jul 07, 2021 at 05:24:08PM +0800, Jason Wang wrote:
> > > 在 2021/7/7 下午4:55, Stefan Hajnoczi 写道:
> > > > On Wed, Jul 07, 2021 at 11:43:28AM +0800, Jason Wang wrote:
> > > > > 在 2021/7/7 上午1:11, Stefan Hajnoczi 写道:
> > > > > > On Tue, Jul 06, 2021 at 09:08:26PM +0800, Jason Wang wrote:
> > > > > > > On Tue, Jul 6, 2021 at 6:15 PM Stefan Hajnoczi <stefanha@xxxxxxxxxx> wrote:
> > > > > > > > On Tue, Jul 06, 2021 at 10:34:33AM +0800, Jason Wang wrote:
> > > > > > > > > 在 2021/7/5 下午8:49, Stefan Hajnoczi 写道:
> > > > > > > > > > On Mon, Jul 05, 2021 at 11:36:15AM +0800, Jason Wang wrote:
> > > > > > > > > > > 在 2021/7/4 下午5:49, Yongji Xie 写道:
> > > > > > > > > > > > > > OK, I get you now. Since the VIRTIO specification says "Device
> > > > > > > > > > > > > > configuration space is generally used for rarely-changing or
> > > > > > > > > > > > > > initialization-time parameters". I assume the VDUSE_DEV_SET_CONFIG
> > > > > > > > > > > > > > ioctl should not be called frequently.
> > > > > > > > > > > > > The spec uses MUST and other terms to define the precise requirements.
> > > > > > > > > > > > > Here the language (especially the word "generally") is weaker and means
> > > > > > > > > > > > > there may be exceptions.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > Another type of access that doesn't work with the VDUSE_DEV_SET_CONFIG
> > > > > > > > > > > > > approach is reads that have side-effects. For example, imagine a field
> > > > > > > > > > > > > containing an error code if the device encounters a problem unrelated to
> > > > > > > > > > > > > a specific virtqueue request. Reading from this field resets the error
> > > > > > > > > > > > > code to 0, saving the driver an extra configuration space write access
> > > > > > > > > > > > > and possibly race conditions. It isn't possible to implement those
> > > > > > > > > > > > > semantics suing VDUSE_DEV_SET_CONFIG. It's another corner case, but it
> > > > > > > > > > > > > makes me think that the interface does not allow full VIRTIO semantics.
> > > > > > > > > > > Note that though you're correct, my understanding is that config space is
> > > > > > > > > > > not suitable for this kind of error propagating. And it would be very hard
> > > > > > > > > > > to implement such kind of semantic in some transports.  Virtqueue should be
> > > > > > > > > > > much better. As Yong Ji quoted, the config space is used for
> > > > > > > > > > > "rarely-changing or intialization-time parameters".
> > > > > > > > > > > 
> > > > > > > > > > > 
> > > > > > > > > > > > Agreed. I will use VDUSE_DEV_GET_CONFIG in the next version. And to
> > > > > > > > > > > > handle the message failure, I'm going to add a return value to
> > > > > > > > > > > > virtio_config_ops.get() and virtio_cread_* API so that the error can
> > > > > > > > > > > > be propagated to the virtio device driver. Then the virtio-blk device
> > > > > > > > > > > > driver can be modified to handle that.
> > > > > > > > > > > > 
> > > > > > > > > > > > Jason and Stefan, what do you think of this way?
> > > > > > > > > > Why does VDUSE_DEV_GET_CONFIG need to support an error return value?
> > > > > > > > > > 
> > > > > > > > > > The VIRTIO spec provides no way for the device to report errors from
> > > > > > > > > > config space accesses.
> > > > > > > > > > 
> > > > > > > > > > The QEMU virtio-pci implementation returns -1 from invalid
> > > > > > > > > > virtio_config_read*() and silently discards virtio_config_write*()
> > > > > > > > > > accesses.
> > > > > > > > > > 
> > > > > > > > > > VDUSE can take the same approach with
> > > > > > > > > > VDUSE_DEV_GET_CONFIG/VDUSE_DEV_SET_CONFIG.
> > > > > > > > > > 
> > > > > > > > > > > I'd like to stick to the current assumption thich get_config won't fail.
> > > > > > > > > > > That is to say,
> > > > > > > > > > > 
> > > > > > > > > > > 1) maintain a config in the kernel, make sure the config space read can
> > > > > > > > > > > always succeed
> > > > > > > > > > > 2) introduce an ioctl for the vduse usersapce to update the config space.
> > > > > > > > > > > 3) we can synchronize with the vduse userspace during set_config
> > > > > > > > > > > 
> > > > > > > > > > > Does this work?
> > > > > > > > > > I noticed that caching is also allowed by the vhost-user protocol
> > > > > > > > > > messages (QEMU's docs/interop/vhost-user.rst), but the device doesn't
> > > > > > > > > > know whether or not caching is in effect. The interface you outlined
> > > > > > > > > > above requires caching.
> > > > > > > > > > 
> > > > > > > > > > Is there a reason why the host kernel vDPA code needs to cache the
> > > > > > > > > > configuration space?
> > > > > > > > > Because:
> > > > > > > > > 
> > > > > > > > > 1) Kernel can not wait forever in get_config(), this is the major difference
> > > > > > > > > with vhost-user.
> > > > > > > > virtio_cread() can sleep:
> > > > > > > > 
> > > > > > > >      #define virtio_cread(vdev, structname, member, ptr)                     \
> > > > > > > >              do {                                                            \
> > > > > > > >                      typeof(((structname*)0)->member) virtio_cread_v;        \
> > > > > > > >                                                                              \
> > > > > > > >                      might_sleep();                                          \
> > > > > > > >                      ^^^^^^^^^^^^^^
> > > > > > > > 
> > > > > > > > Which code path cannot sleep?
> > > > > > > Well, it can sleep but it can't sleep forever. For VDUSE, a
> > > > > > > buggy/malicious userspace may refuse to respond to the get_config.
> > > > > > > 
> > > > > > > It looks to me the ideal case, with the current virtio spec, for VDUSE is to
> > > > > > > 
> > > > > > > 1) maintain the device and its state in the kernel, userspace may sync
> > > > > > > with the kernel device via ioctls
> > > > > > > 2) offload the datapath (virtqueue) to the userspace
> > > > > > > 
> > > > > > > This seems more robust and safe than simply relaying everything to
> > > > > > > userspace and waiting for its response.
> > > > > > > 
> > > > > > > And we know for sure this model can work, an example is TUN/TAP:
> > > > > > > netdevice is abstracted in the kernel and datapath is done via
> > > > > > > sendmsg()/recvmsg().
> > > > > > > 
> > > > > > > Maintaining the config in the kernel follows this model and it can
> > > > > > > simplify the device generation implementation.
> > > > > > > 
> > > > > > > For config space write, it requires more thought but fortunately it's
> > > > > > > not commonly used. So VDUSE can choose to filter out the
> > > > > > > device/features that depends on the config write.
> > > > > > This is the problem. There are other messages like SET_FEATURES where I
> > > > > > guess we'll face the same challenge.
> > > > > Probably not, userspace device can tell the kernel about the device_features
> > > > > and mandated_features during creation, and the feature negotiation could be
> > > > > done purely in the kernel without bothering the userspace.
> > > 
> > > (For some reason I drop the list accidentally, adding them back, sorry)
> > > 
> > > 
> > > > Sorry, I confused the messages. I meant SET_STATUS. It's a synchronous
> > > > interface where the driver waits for the device.
> > > 
> > > It depends on how we define "synchronous" here. If I understand correctly,
> > > the spec doesn't expect there will be any kind of failure for the operation
> > > of set_status itself.
> > > 
> > > Instead, anytime it want any synchronization, it should be done via
> > > get_status():
> > > 
> > > 1) re-read device status to make sure FEATURES_OK is set during feature
> > > negotiation
> > > 2) re-read device status to be 0 to make sure the device has finish the
> > > reset
> > > 
> > > 
> > > > VDUSE currently doesn't wait for the device emulation process to handle
> > > > this message (no reply is needed) but I think this is a mistake because
> > > > VDUSE is not following the VIRTIO device model.
> > > 
> > > With the trick that is done for FEATURES_OK above, I think we don't need to
> > > wait for the reply.
> > > 
> > > If userspace takes too long to respond, it can be detected since
> > > get_status() doesn't return the expected value for long time.
> > > 
> > > And for the case that needs a timeout, we probably can use NEEDS_RESET.
> > I think you're right. get_status is the synchronization point, not
> > set_status.
> > 
> > Currently there is no VDUSE GET_STATUS message. The
> > VDUSE_START/STOP_DATAPLANE messages could be changed to SET_STATUS so
> > that the device emulation program can participate in emulating the
> > Device Status field.
> 
> 
> I'm not sure I get this, but it is what has been done?
> 
> +static void vduse_vdpa_set_status(struct vdpa_device *vdpa, u8 status)
> +{
> +    struct vduse_dev *dev = vdpa_to_vduse(vdpa);
> +    bool started = !!(status & VIRTIO_CONFIG_S_DRIVER_OK);
> +
> +    dev->status = status;
> +
> +    if (dev->started == started)
> +        return;
> +
> +    dev->started = started;
> +    if (dev->started) {
> +        vduse_dev_start_dataplane(dev);
> +    } else {
> +        vduse_dev_reset(dev);
> +        vduse_dev_stop_dataplane(dev);
> +    }
> +}
> 
> 
> But the looks not correct:
> 
> 1) !DRIVER_OK doesn't means a reset?
> 2) Need to deal with FEATURES_OK

I'm not sure if this reply was to me or to Yongji Xie?

Currently vduse_vdpa_set_status() does not allow the device emulation
program to participate fully in Device Status field changes. It hides
the status bits and only sends VDUSE_START/STOP_DATAPLANE.

I suggest having GET_STATUS/SET_STATUS messages instead, allowing the
device emulation program to handle these parts of the VIRTIO device
model (e.g. rejecting combinations of features that are mutually
exclusive).

Stefan

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