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

From: Halil Pasic
Date: Fri Feb 21 2020 - 08:12:44 EST

On Thu, 20 Feb 2020 15:55:14 -0500
"Michael S. Tsirkin" <mst@xxxxxxxxxx> 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.

Thanks for your constructive approach. I do want the hypervisor to
assume it gets GPA's. My train of thought was that the guys that need
to use IOVA's that are not GPA's when force_dma_unencrypted() will have
to to specify VIRTIO_F_ACCESS_PLATFORM (at the device) anyway, because
otherwise it won't work. But I see your point: in case of a
mis-configuration and provided the DMA API returns IOVA's one could end
up trying to touch wrong memory locations. But this should be similar to
what would happen if DMA ops are not used, and memory is not made accessible.

> 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.

Unnecessary for powerpc because they do normal PCI. In the context of
CCW there are only guest physical addresses (CCW I/O has no concept of

> 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

force_dma_unencrypted(&vdev->dev) is IMHO exactly about this.

> - DMA API is giving us addresses that are actually also physical
> addresses

In case of s390 this is given. I talked with the power people before
posting this, and they ensured me they can are willing to deal with
this. I was hoping to talk abut this with the AMD SEV people here (hence
the cc).

> - Hypervisor is buggy and didn't enable VIRTIO_F_ACCESS_PLATFORM

I don't get this point. The argument where the hypervisor is buggy is a
bit hard to follow for me. If hypervisor is buggy we have already lost
anyway most of the time, or?

> I don't see how this patch does this.

I do get your point. I don't know of a good way to check that DMA API
is giving us addresses that are actually physical addresses, and the
situation you describe definitely has some risk to it.

Let me comment on other ideas that came up. I would be very happy to go
with the best one. Thank you very much.


> > ---
> > 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