On Tue, Jan 29, 2019 at 09:36:08PM -0500, Michael S. Tsirkin wrote:

This has been discussed ad nauseum. virtio is all about compatibility.
Losing a couple of lines of code isn't worth breaking working setups.
People that want "just use DMA API no tricks" now have the option.
Setting a flag in a feature bit map is literally a single line
of code in the hypervisor. So stop pushing for breaking working
legacy setups and just fix it in the right place.

I agree with the legacy aspect.  What I am missing is an extremely
strong wording that says you SHOULD always set this flag for new
hosts, including an explanation why.