On 2020/2/24 下午2:06, Michael S. Tsirkin wrote: > On Mon, Feb 24, 2020 at 12:01:57PM +0800, Jason Wang wrote: >> On 2020/2/21 下午10:56, Halil Pasic wrote: >>> On Fri, 21 Feb 2020 14:22:26 +0800 >>> Jason Wang 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. >>> >>> 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! >> >> Using IOTLB API on platform without IOMMU support is not intended. Please >> try the attached patch to see if it helps. >> >> Thanks >> >> >>> 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 >> >From 66fa730460875ac99e81d7db2334cd16bb1d2b27 Mon Sep 17 00:00:00 2001 >> From: Jason Wang >> Date: Mon, 24 Feb 2020 12:00:10 +0800 >> Subject: [PATCH] virtio: turn on IOMMU_PLATFORM properly >> >> When transport does not support IOMMU, we should clear IOMMU_PLATFORM >> even if the device and vhost claims to support that. This help to >> avoid the performance overhead caused by unnecessary IOTLB miss/update >> transactions on such platform. >> >> Signed-off-by: Jason Wang >> --- >> hw/virtio/virtio-bus.c | 6 +++--- >> 1 file changed, 3 insertions(+), 3 deletions(-) >> >> diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c >> index d6332d45c3..2741b9fdd2 100644 >> --- a/hw/virtio/virtio-bus.c >> +++ b/hw/virtio/virtio-bus.c >> @@ -47,7 +47,6 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp) >> VirtioBusState *bus = VIRTIO_BUS(qbus); >> VirtioBusClass *klass = VIRTIO_BUS_GET_CLASS(bus); >> VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev); >> - bool has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM); >> Error *local_err = NULL; >> >> DPRINTF("%s: plug device.\n", qbus->name); >> @@ -77,10 +76,11 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp) >> return; >> } >> >> - if (klass->get_dma_as != NULL && has_iommu) { >> - virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM); >> + if (false && klass->get_dma_as != NULL && >> + virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) { >> vdev->dma_as = klass->get_dma_as(qbus->parent); >> } else { >> + virtio_clear_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM); >> vdev->dma_as = &address_space_memory; >> } >> } > > This seems to clear it unconditionally. I guess it's just a debugging > patch, the real one will come later? My bad, here's the correct one. Thanks > >> -- >> 2.19.1 >>