On Mon, Jan 20, 2020 at 09:47:18PM +0000, Shahaf Shuler wrote:

Monday, January 20, 2020 11:25 PM, Michael S. Tsirkin:

quoted

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

On Mon, Jan 20, 2020 at 08:51:43PM +0000, Shahaf Shuler wrote:

quoted

Monday, January 20, 2020 7:50 PM, Jason Gunthorpe:

quoted

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

On Mon, Jan 20, 2020 at 04:43:53PM +0800, Jason Wang wrote:

quoted

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().

quoted

quoted

quoted

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.

+1.
It will be best to have one API for static config (i.e. mapping can be
set only before virtio device gets active), and one API for dynamic
changes that can be set after the virtio device is active.

Frankly I don't see when we'd use the static one.
Memory hotplug is enabled for most guests...

The fact memory hotplug is enabled doesn't necessarily means there is not cold-plugged memory on the hot plugged slots. 
So your claim is majority of guests are deployed w/o any cold-plugged memory? 

Sorry for not being clear. I was merely saying that dynamic one
can't be optional, and static one can. So how about we
start just with the dynamic one, then add the static one
as a later optimization?

quoted

quoted

quoted

quoted

quoted

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