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

[Michael S. Tsirkin]

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.
> 
> But VIRTIO_F_IOMMU_PLATFORM a.k.a. VIRTIO_F_ACCESS_PLATFORM is really a
> 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
    addresses
  - 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