Virtualization - Dec 2020 - [RFC v2 00/13] Introduce VDUSE - vDPA Device in Userspace

On 2020/12/23 ??6:59, Yongji Xie wrote:
> On Wed, Dec 23, 2020 at 2:38 PM Jason Wang <jasowang at redhat.com>
wrote:
>>
>> On 2020/12/22 ??10:52, Xie Yongji wrote:
>>> This series introduces a framework, which can be used to implement
>>> vDPA Devices in a userspace program. The work consist of two parts:
>>> control path forwarding and data path offloading.
>>>
>>> In the control path, the VDUSE driver will make use of message
>>> mechnism to forward the config operation from vdpa bus driver
>>> to userspace. Userspace can use read()/write() to receive/reply
>>> those control messages.
>>>
>>> In the data path, the core is mapping dma buffer into VDUSE
>>> daemon's address space, which can be implemented in different
ways
>>> depending on the vdpa bus to which the vDPA device is attached.
>>>
>>> In virtio-vdpa case, we implements a MMU-based on-chip IOMMU driver
with
>>> bounce-buffering mechanism to achieve that.
>>
>> Rethink about the bounce buffer stuffs. I wonder instead of using
kernel
>> pages with mmap(), how about just use userspace pages like what vhost
did?
>>
>> It means we need a worker to do bouncing but we don't need to care
about
>> annoying stuffs like page reclaiming?
>>
> Now the I/O bouncing is done in the streaming DMA mapping routines
> which can be called from interrupt context. If we put this into a
> kworker, that means we need to synchronize with a kworker in an
> interrupt context. I think it can't work.

We just need to make sure the buffer is ready before the user is trying 
to access them.

But I admit it would be tricky (require shadow virtqueue etc) which is 
probably not a good idea.

Thanks

>
> Thanks,
> Yongji
>