Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted

From: Michael S. Tsirkin
Date: Tue Feb 05 2019 - 11:14:01 EST

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:
> > 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.