Messages in this thread | | | Subject | Re: [PATCH v8 10/10] Documentation: Add documentation for VDUSE | From | Jason Wang <> | Date | Thu, 8 Jul 2021 12:17:56 +0800 |
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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@redhat.com> 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
Thanks
> This change could affect VDUSE's VIRTIO feature > interface since the device emulation program can reject features by not > setting FEATURES_OK. > > Stefan
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