RE: [PATCH 2/2] virtio: let virtio use DMA API when guest RAM is protected

From: Ram Pai
Date: Thu Feb 20 2020 - 20:18:09 EST

On Thu, Feb 20, 2020 at 03:55:14PM -0500, Michael S. Tsirkin wrote:
> On Thu, Feb 20, 2020 at 05:06:06PM +0100, Halil Pasic wrote:
> > Currently the advanced guest memory protection technologies (AMD SEV,
> > powerpc secure guest technology and s390 Protected VMs) abuse the
> > VIRTIO_F_IOMMU_PLATFORM flag to make virtio core use the DMA API, which
> > is in turn necessary, to make IO work with guest memory protection.
> >
> > different beast: with virtio devices whose implementation runs on an SMP
> > CPU we are still fine with doing all the usual optimizations, it is just
> > that we need to make sure that the memory protection mechanism does not
> > get in the way. The VIRTIO_F_ACCESS_PLATFORM mandates more work on the
> > side of the guest (and possibly he host side as well) than we actually
> > need.
> >
> > An additional benefit of teaching the guest to make the right decision
> > (and use DMA API) on it's own is: removing the need, to mandate special
> > VM configuration for guests that may run with protection. This is
> > especially interesting for s390 as VIRTIO_F_IOMMU_PLATFORM pushes all
> > the virtio control structures into the first 2G of guest memory:
> > something we don't necessarily want to do per-default.
> >
> > Signed-off-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
> > Tested-by: Ram Pai <linuxram@xxxxxxxxxx>
> > Tested-by: Michael Mueller <mimu@xxxxxxxxxxxxx>
> This might work for you but it's fragile, since without
> VIRTIO_F_ACCESS_PLATFORM hypervisor assumes it gets
> GPA's, not DMA addresses.
> IOW this looks like another iteration of:
> virtio: Support encrypted memory on powerpc secure guests
> which I was under the impression was abandoned as unnecessary.

It has been abondoned on powerpc. We enabled VIRTIO_F_ACCESS_PLATFORM;
by default, flag on powerpc.

We would like to enable secure guests on powerpc without this flag
aswell enabled, but past experience has educated us that its not a easy
path. However if Halil makes some inroads in this path for s390, we
will like to support him.