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

From: Michael S. Tsirkin
Date: Thu Feb 20 2020 - 15:48:49 EST

On Thu, Feb 20, 2020 at 05:06:04PM +0100, Halil Pasic wrote:
> 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
> 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.

That's just a QEMU thing. It uses the same flag for VIRTIO_F_ACCESS_PLATFORM
and vhost IOTLB. QEMU can separate them, no need to change linux.

> 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