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

[Halil Pasic <pasic@linux.ibm.com>]

On Thu, 20 Feb 2020 16:33:35 -0500
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Thu, Feb 20, 2020 at 05:06:04PM +0100, Halil Pasic wrote:
> > 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.
> 
> So it sounds like a host issue: the emulation of s390 unnecessarily complicated.
> Working around it by the guest looks wrong ...

While do think that forcing IOMMU_PLATFORM on devices that do not
want or need it, just to trigger DMA API usage in guest is conceptually
wrong, I do agree, that we might have a host issue. Namely, unlike PCI,
CCW does not have an IOMMU, and the spec clearly states that "Whether
accesses are actually limited or translated is described by
platform-specific means.". With CCW devices we don't have address translation
by IOMMU, and in that sense vhost is probably wrong about trying to do
the translation. That is why I mentioned the patch, and that it's done
regardless of the transport/platform.

Regards,
Halil


> 
> > 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
>