On Fri, Feb 21, 2020 at 03:56:02PM +0100, Halil Pasic wrote: > On Fri, 21 Feb 2020 14:22:26 +0800 > Jason Wang <jasowang@xxxxxxxxxx> wrote: > > > > > On 2020/2/21 上午12:06, 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). > > > > > > Do you see a consistent degradation on the performance, or it only > > happen when for during the beginning of the test? > > > > AFAIK the degradation is consistent. > > > > > > BTW with ccw I/O there is > > > (architecturally) no IOMMU, so the whole address translation makes no > > > sense in the context of virtio-ccw. > > > > > > I suspect we can do optimization in qemu side. > > > > E.g send memtable entry via IOTLB API when vIOMMU is not enabled. > > > > If this makes sense, I can draft patch to see if there's any difference. > > Frankly I would prefer to avoid IOVAs on the descriptor ring (and the > then necessary translation) for virtio-ccw altogether. But Michael > voiced his opinion that we should mandate F_IOMMU_PLATFORM for devices > that could be used with guests running in protected mode. I don't share > his opinion, but that's an ongoing discussion. I'm a bit confused by this. For the ccw specific case, F_ACCESS_PLATFORM shouldn't have any impact: for you, IOVA == GPA so everything is easy. > Should we end up having to do translation from IOVA in vhost, we are > very interested in that translation being fast and efficient. > > In that sense we would be very happy to test any optimization that aim > into that direction. > > Thank you very much for your input! > > Regards, > Halil > > > > > Thanks > > > > > > > > > > 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 > > > -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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