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

From: Michael S. Tsirkin
Date: Thu Feb 20 2020 - 15:55:28 EST

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.

To summarize, the necessary conditions for a hack along these lines
(using DMA API without VIRTIO_F_ACCESS_PLATFORM) are that we detect that:

- secure guest mode is enabled - so we know that since we don't share
most memory regular virtio code won't
work, even though the buggy hypervisor didn't set VIRTIO_F_ACCESS_PLATFORM
- DMA API is giving us addresses that are actually also physical
- Hypervisor is buggy and didn't enable VIRTIO_F_ACCESS_PLATFORM

I don't see how this patch does this.

> ---
> drivers/virtio/virtio_ring.c | 3 +++
> 1 file changed, 3 insertions(+)
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 867c7ebd3f10..fafc8f924955 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -243,6 +243,9 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
> if (!virtio_has_iommu_quirk(vdev))
> return true;
> + if (force_dma_unencrypted(&vdev->dev))
> + return true;
> +
> /* Otherwise, we are left to guess. */
> /*
> * In theory, it's possible to have a buggy QEMU-supposed
> --
> 2.17.1