On Tue, Feb 05, 2019 at 08:24:07AM +0100, Christoph Hellwig wrote:

On Mon, Feb 04, 2019 at 04:38:21PM -0500, Michael S. Tsirkin wrote:

quoted

It was designed to make, when set, as many guests as we can work
correctly, and it seems to be successful in doing exactly that.

Unfortunately there could be legacy guests that do work correctly but
become slow. Whether trying to somehow work around that
can paint us into a corner where things again don't
work for some people is a question worth discussing.

The other problem is that some qemu machines just throw passthrough
devices and virtio devices on the same virtual PCI(e) bus, and have a
common IOMMU setup for the whole bus / root port / domain.  I think
this is completely bogus, but unfortunately it is out in the field.

Given that power is one of these examples I suspect that is what
Thiago referes to.  But in this case the answer can't be that we
pile on hack ontop of another, but instead introduce a new qemu
machine that separates these clearly, and make that mandatory for
the secure guest support.

That could we one approach, assuming one exists that guests
already support.

-- 
MST