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 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