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

[Halil Pasic <pasic@linux.ibm.com>]

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
expressed by VIRTIO_F_IOMMU_PLATFORM). 

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
VIRTIO_F_IOMMU_PLATFORM'.

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
-- 
2.17.1

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