Re: [RFC v3 00/29] vDPA software assisted live migration

[Michael S. Tsirkin]

On Wed, May 19, 2021 at 06:28:34PM +0200, Eugenio Pérez wrote:
> Commit 17 introduces the buffer forwarding. Previous one are for
> preparations again, and laters are for enabling some obvious
> optimizations. However, it needs the vdpa device to be able to map
> every IOVA space, and some vDPA devices are not able to do so. Checking
> of this is added in previous commits.

That might become a significant limitation. And it worries me that
this is such a big patchset which might yet take a while to get
finalized.

I have an idea: how about as a first step we implement a transparent
switch from vdpa to a software virtio in QEMU or a software vhost in
kernel?

This will give us live migration quickly with performance comparable
to failover but without dependance on guest cooperation.

Next step could be driving vdpa from userspace while still copying
packets to a pre-registered buffer.

Finally your approach will be a performance optimization for devices
that support arbitrary IOVA.

Thoughts?

-- 
MST