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 > 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. 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
