[PATCH 0/2] virtio: decouple protected guest RAM form VIRTIO_F_IOMMU_PLATFORM

From: Halil Pasic
Date: Thu Feb 20 2020 - 11:08:03 EST

Currently if one intends to run a memory protection enabled VM with
virtio devices and linux as the guest OS, one needs to specify the
VIRTIO_F_IOMMU_PLATFORM flag for each virtio device to make the guest
linux use the DMA API, which in turn handles the memory
encryption/protection stuff if the guest decides to turn itself into
a protected one. This however makes no sense due to multiple reasons:
* The device is not changed by the fact that the guest RAM is
protected. The so called IOMMU bypass quirk is not affected.
* This usage is not congruent with standardised semantics of
VIRTIO_F_IOMMU_PLATFORM. Guest memory protected is an orthogonal reason
for using DMA API in virtio (orthogonal with respect to what is

This series aims to decouple 'have to use DMA API because my (guest) RAM
is protected' and 'have to use DMA API because the device told me

Please find more detailed explanations about the conceptual aspects in
the individual patches. There is however also a very practical problem
that is addressed by this series.

For vhost-net the feature VIRTIO_F_IOMMU_PLATFORM has the following side
effect The vhost code assumes it the addresses on the virtio descriptor
ring are not guest physical addresses but iova's, and insists on doing a
translation of these regardless of what transport is used (e.g. whether
we emulate a PCI or a CCW device). (For details see commit 6b1e6cc7855b
"vhost: new device IOTLB API".) On s390 this results in severe
performance degradation (c.a. factor 10). BTW with ccw I/O there is
(architecturally) no IOMMU, so the whole address translation makes no
sense in the context of virtio-ccw.

Halil Pasic (2):
mm: move force_dma_unencrypted() to mem_encrypt.h
virtio: let virtio use DMA API when guest RAM is protected

drivers/virtio/virtio_ring.c | 3 +++
include/linux/dma-direct.h | 9 ---------
include/linux/mem_encrypt.h | 10 ++++++++++
3 files changed, 13 insertions(+), 9 deletions(-)

base-commit: ca7e1fd1026c5af6a533b4b5447e1d2f153e28f2