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Subject: Re: [RFC] virtio: Use DMA MAP API for devices without an IOMMU
From:   Benjamin Herrenschmidt <benh@kernel.crashing.org>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Anshuman Khandual <khandual@linux.vnet.ibm.com>,
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        david@gibson.dropbear.id.au
Date:   Fri, 06 Apr 2018 07:18:33 +1000
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On Thu, 2018-04-05 at 21:34 +0300, Michael S. Tsirkin wrote:
> > In this specific case, because that would make qemu expect an iommu,
> > and there isn't one.
> 
> 
> I think that you can set iommu_platform in qemu without an iommu.

No I mean the platform has one but it's not desirable for it to be used
due to the performance hit.

Cheers,
Ben.
> 
> > Anshuman, you need to provide more background here. I don't have time
> > right now it's late, but explain about the fact that this is for a
> > specific type of secure VM which has only a limited pool of (insecure)
> > memory that can be shared with qemu, so all IOs need to bounce via that
> > pool, which can be achieved by using swiotlb.
> > 
> > Note: this isn't urgent, we can discuss alternative approaches, this is
> > just to start the conversation.
> > 
> > Cheers,
> > Ben.