Re: [PATCH 3/5] vDPA: introduce vDPA bus

Jason Gunthorpe
2020-01-20T17:49:49Z

On Mon, Jan 20, 2020 at 04:43:53PM +0800, Jason Wang wrote:
> This is similar to the design of platform IOMMU part of vhost-vdpa. We
> decide to send diffs to platform IOMMU there. If it's ok to do that in
> driver, we can replace set_map with incremental API like map()/unmap().
>
> Then driver need to maintain rbtree itself.

I think we really need to see two modes, one where there is a fixed
translation without dynamic vIOMMU driven changes and one that
supports vIOMMU.

There are different optimization goals in the drivers for these two
configurations.

> > If the first one, then I think memory hotplug is a heavy flow
> > regardless. Do you think the extra cycles for the tree traverse
> > will be visible in any way?
>
> I think if the driver can pause the DMA during the time for setting up new
> mapping, it should be fine.

This is very tricky for any driver if the mapping change hits the
virtio rings. :(

Even a IOMMU using driver is going to have problems with that..

Jason