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

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

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

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

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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.

+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...

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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