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

[Michael S. Tsirkin]

On Mon, Feb 24, 2020 at 12:01:57PM +0800, Jason Wang wrote:
> 
> On 2020/2/21 äå10:56, Halil Pasic wrote:
> > On Fri, 21 Feb 2020 14:22:26 +0800
> > Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > 
> > > On 2020/2/21 äå12:06, 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 <jasowang@xxxxxxxxxx>
> 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 <jasowang@xxxxxxxxxx>
> ---
>  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?

> -- 
> 2.19.1
>