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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
