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SubjectRe: [PATCH V4 3/5] vDPA: introduce vDPA bus
Date
On Thu, Feb 20, 2020 at 02:11:39PM +0800, Jason Wang wrote:
> vDPA device is a device that uses a datapath which complies with the
> virtio specifications with vendor specific control path. vDPA devices
> can be both physically located on the hardware or emulated by
> software. vDPA hardware devices are usually implemented through PCIE
> with the following types:
>
> - PF (Physical Function) - A single Physical Function
> - VF (Virtual Function) - Device that supports single root I/O
> virtualization (SR-IOV). Its Virtual Function (VF) represents a
> virtualized instance of the device that can be assigned to different
> partitions
> - ADI (Assignable Device Interface) and its equivalents - With
> technologies such as Intel Scalable IOV, a virtual device (VDEV)
> composed by host OS utilizing one or more ADIs. Or its equivalent
> like SF (Sub function) from Mellanox.
>
> From a driver's perspective, depends on how and where the DMA
> translation is done, vDPA devices are split into two types:
>
> - Platform specific DMA translation - From the driver's perspective,
> the device can be used on a platform where device access to data in
> memory is limited and/or translated. An example is a PCIE vDPA whose
> DMA request was tagged via a bus (e.g PCIE) specific way. DMA
> translation and protection are done at PCIE bus IOMMU level.
> - Device specific DMA translation - The device implements DMA
> isolation and protection through its own logic. An example is a vDPA
> device which uses on-chip IOMMU.
>
> To hide the differences and complexity of the above types for a vDPA
> device/IOMMU options and in order to present a generic virtio device
> to the upper layer, a device agnostic framework is required.
>
> This patch introduces a software vDPA bus which abstracts the
> common attributes of vDPA device, vDPA bus driver and the
> communication method (vdpa_config_ops) between the vDPA device
> abstraction and the vDPA bus driver. This allows multiple types of
> drivers to be used for vDPA device like the virtio_vdpa and vhost_vdpa
> driver to operate on the bus and allow vDPA device could be used by
> either kernel virtio driver or userspace vhost drivers as:
>
> virtio drivers vhost drivers
> | |
> [virtio bus] [vhost uAPI]
> | |
> virtio device vhost device
> virtio_vdpa drv vhost_vdpa drv
> \ /
> [vDPA bus]
> |
> vDPA device
> hardware drv
> |
> [hardware bus]
> |
> vDPA hardware

I still don't like this strange complexity, vhost should have been
layered on top of the virtio device instead of adding an extra bus
just for vdpa.

However, I don't see any technical problems with this patch now.

Thanks,
Jason

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