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

From: Jason Wang
Date: Sun Feb 23 2020 - 23:02:24 EST

This is a multi-part message in MIME format.
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

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

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.




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