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

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
>>> 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.


> 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)

- 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;
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