On 2020/10/20 下午4:35, Yongji Xie wrote:
On Tue, Oct 20, 2020 at 4:01 PM Jason Wang <jasowang@xxxxxxxxxx
<mailto:jasowang@xxxxxxxxxx>> wrote:
On 2020/10/20 下午3:39, Yongji Xie wrote:
>
>
> On Tue, Oct 20, 2020 at 11:20 AM Jason Wang <jasowang@xxxxxxxxxx
<mailto:jasowang@xxxxxxxxxx>
> <mailto:jasowang@xxxxxxxxxx <mailto:jasowang@xxxxxxxxxx>>> wrote:
>
>
> 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?
I'm not sure I get this, any reason that you want to use
vm_insert_page() to VM's memory. Or do you mean you want to implement
some kind of zero-copy?
If my understanding is right, we will have a QEMU (VM) process and a
device emulation process in the vhost-vdpa case, right? When I/O
happens, the virtio driver in VM will put the IOVA to vring and device
emulation process will get the IOVA from vring. Then the device
emulation process will translate the IOVA to its VA to access the dma
buffer which resides in VM's memory. That means the device emulation
process needs to access VM's memory, so we should use vm_insert_page()
to build the page table of the device emulation process.
Ok, I get you now. So it looks to me the that the real issue is not the
limitation to anonymous page but see the comments above vm_insert_page():
"
* The page has to be a nice clean _individual_ kernel allocation.
"
So I suspect that using vm_insert_page() to share pages between
processes is legal. We need inputs from MM experts.
Thanks
I guess from the software device implemention in user space it
only need
to receive IOVA ranges and map them in its own address space.
How to map them in its own address space if we don't use vm_insert_page()?
