On 2020/10/19 下午10:56, Xie Yongji wrote:
> This series introduces a framework, which can be used to implement
> vDPA Devices in a userspace program. To implement it, the work
> consist of two parts: control path emulating and data path offloading.
>
> In the control path, the VDUSE driver will make use of message
> mechnism to forward the actions (get/set features, get/st status,
> get/set config space and set virtqueue states) from virtio-vdpa
> driver to userspace. Userspace can use read()/write() to
> receive/reply to those control messages.
>
> In the data path, the VDUSE driver implements a MMU-based
> on-chip IOMMU driver which supports both direct mapping and
> indirect mapping with bounce buffer. Then userspace can access
> those iova space via mmap(). Besides, eventfd mechnism is used to
> trigger interrupts and forward virtqueue kicks.
This is pretty interesting!
For vhost-vdpa, it should work, but for virtio-vdpa, I think we should
carefully deal with the IOMMU/DMA ops stuffs.
I notice that neither dma_map nor set_map is implemented in
vduse_vdpa_config_ops, this means you want to let vhost-vDPA to deal
with IOMMU domains stuffs. Any reason for doing that?
Actually, this series
only focus on virtio-vdpa case now. To support vhost-vdpa, as you said, we need to implement dma_map/dma_unmap. But there is a limit that vm's memory can't be anonymous pages which are forbidden in vm_insert_page(). Maybe we need to add some limits on vhost-vdpa?
The reason for the questions are:
1) You've implemented a on-chip IOMMU driver but don't expose it to
generic IOMMU layer (or generic IOMMU layer may need some extension to
support this)
2) We will probably remove the IOMMU domain management in vhost-vDPA,
and move it to the device(parent).
So if it's possible, please implement either set_map() or
dma_map()/dma_unmap(), this may align with our future goal and may speed
up the development.
Btw, it would be helpful to give even more details on how the on-chip
IOMMU driver in implemented.
The basic idea is treating MMU (VA->PA) as IOMMU (IOVA->PA). And using vm_insert_page()/zap_page_range() to do address mapping/unmapping. And the address mapping will be done in page fault handler because vm_insert_page() can't be called in atomic_context such as dma_map_ops->map_page().
>
> The details and our user case is shown below:
>
> ------------------------ -----------------------------------------------------------
> | APP | | QEMU |
> | --------- | | -------------------- -------------------+<-->+------ |
> | |dev/vdx| | | | device emulation | | virtio dataplane | | BDS | |
> ------------+----------- -----------+-----------------------+-----------------+-----
> | | | |
> | | emulating | offloading |
> ------------+---------------------------+-----------------------+-----------------+------
> | | block device | | vduse driver | | vdpa device | | TCP/IP | |
> | -------+-------- --------+-------- +------+------- -----+---- |
> | | | | | | |
> | | | | | | |
> | ----------+---------- ----------+----------- | | | |
> | | virtio-blk driver | | virtio-vdpa driver | | | | |
> | ----------+---------- ----------+----------- | | | |
> | | | | | | |
> | | ------------------ | | |
> | ----------------------------------------------------- ---+--- |
> ------------------------------------------------------------------------------ | NIC |---
> ---+---
> |
> ---------+---------
> | Remote Storages |
> -------------------
The figure is not very clear to me in the following points:
1) if the device emulation and virtio dataplane is all implemented in
QEMU, what's the point of doing this? I thought the device should be a
remove process?
2) it would be better to draw a vDPA bus somewhere to help people to
understand the architecture
3) for the "offloading" I guess it should be done virtio vhost-vDPA, so
it's better to draw a vhost-vDPA block there
This figure only shows virtio-vdpa case, I will take vhost-vdpa case into consideration in next version.
Thanks,
Yongji