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Date:   Thu, 20 Feb 2020 16:33:35 -0500
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Halil Pasic <pasic@linux.ibm.com>
Cc:     Jason Wang <jasowang@redhat.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Christoph Hellwig <hch@lst.de>, linux-s390@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Viktor Mihajlovski <mihajlov@linux.ibm.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Ram Pai <linuxram@us.ibm.com>,
        Thiago Jung Bauermann <bauerman@linux.ibm.com>,
        David Gibson <david@gibson.dropbear.id.au>,
        "Lendacky, Thomas" <Thomas.Lendacky@amd.com>,
        Michael Mueller <mimu@linux.ibm.com>
Subject: Re: [PATCH 0/2] virtio: decouple protected guest RAM form
 VIRTIO_F_IOMMU_PLATFORM
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References: <20200220160606.53156-1-pasic@linux.ibm.com>
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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 ...

> 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