Messages in this thread | | | Subject | Re: [RFC v4 0/3] vhost: introduce mdev based hardware backend | From | Jason Wang <> | Date | Fri, 20 Sep 2019 09:30:58 +0800 |
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On 2019/9/19 下午11:45, Tiwei Bie wrote: > On Thu, Sep 19, 2019 at 09:08:11PM +0800, Jason Wang wrote: >> On 2019/9/18 下午10:32, Michael S. Tsirkin wrote: >>>>>> So I have some questions: >>>>>> >>>>>> 1) Compared to method 2, what's the advantage of creating a new vhost char >>>>>> device? I guess it's for keep the API compatibility? >>>>> One benefit is that we can avoid doing vhost ioctls on >>>>> VFIO device fd. >>>> Yes, but any benefit from doing this? >>> It does seem a bit more modular, but it's certainly not a big deal. >> Ok, if we go this way, it could be as simple as provide some callback to >> vhost, then vhost can just forward the ioctl through parent_ops. >> >>>>>> 2) For method 2, is there any easy way for user/admin to distinguish e.g >>>>>> ordinary vfio-mdev for vhost from ordinary vfio-mdev? >>>>> I think device-api could be a choice. >>>> Ok. >>>> >>>> >>>>>> I saw you introduce >>>>>> ops matching helper but it's not friendly to management. >>>>> The ops matching helper is just to check whether a given >>>>> vfio-device is based on a mdev device. >>>>> >>>>>> 3) A drawback of 1) and 2) is that it must follow vfio_device_ops that >>>>>> assumes the parameter comes from userspace, it prevents support kernel >>>>>> virtio drivers. >>>>>> >>>>>> 4) So comes the idea of method 3, since it register a new vhost-mdev driver, >>>>>> we can use device specific ops instead of VFIO ones, then we can have a >>>>>> common API between vDPA parent and vhost-mdev/virtio-mdev drivers. >>>>> As the above draft shows, this requires introducing a new >>>>> VFIO device driver. I think Alex's opinion matters here. >> Just to clarify, a new type of mdev driver but provides dummy >> vfio_device_ops for VFIO to make container DMA ioctl work. > I see. Thanks! IIUC, you mean we can provide a very tiny > VFIO device driver in drivers/vhost/mdev.c, e.g.: > > static int vfio_vhost_mdev_open(void *device_data) > { > if (!try_module_get(THIS_MODULE)) > return -ENODEV; > return 0; > } > > static void vfio_vhost_mdev_release(void *device_data) > { > module_put(THIS_MODULE); > } > > static const struct vfio_device_ops vfio_vhost_mdev_dev_ops = { > .name = "vfio-vhost-mdev", > .open = vfio_vhost_mdev_open, > .release = vfio_vhost_mdev_release, > }; > > static int vhost_mdev_probe(struct device *dev) > { > struct mdev_device *mdev = to_mdev_device(dev); > > ... Check the mdev device_id proposed in ... > ... https://lkml.org/lkml/2019/9/12/151 ...
To clarify, this should be done through the id_table fields in vhost_mdev_driver, and it should claim it supports virtio-mdev device only:
static struct mdev_class_id id_table[] = { { MDEV_ID_VIRTIO }, { 0 }, };
static struct mdev_driver vhost_mdev_driver = { ... .id_table = id_table, }
> > return vfio_add_group_dev(dev, &vfio_vhost_mdev_dev_ops, mdev);
And in vfio_vhost_mdev_ops, all its need is to just implement vhost-net ioctl and translate them to virtio-mdev transport (e.g device_ops I proposed or ioctls other whatever other method) API. And it could have a dummy ops implementation for the other device_ops.
> } > > static void vhost_mdev_remove(struct device *dev) > { > vfio_del_group_dev(dev); > } > > static struct mdev_driver vhost_mdev_driver = { > .name = "vhost_mdev", > .probe = vhost_mdev_probe, > .remove = vhost_mdev_remove, > }; > > So we can bind above mdev driver to the virtio-mdev compatible > mdev devices when we want to use vhost-mdev. > > After binding above driver to the mdev device, we can setup IOMMU > via VFIO and get VFIO device fd of this mdev device, and pass it > to vhost fd (/dev/vhost-mdev) with a SET_BACKEND ioctl.
Then what vhost-mdev char device did is just forwarding ioctl back to this vfio device fd which seems a overkill. It's simpler that just do ioctl on the device ops directly.
Thanks
> > Thanks, > Tiwei > >> Thanks >> >> >>>> Yes, it is. >>>> >>>> Thanks >>>> >>>>
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