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Date:   Sun, 5 Aug 2018 00:29:30 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc:     Christoph Hellwig <hch@infradead.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Anshuman Khandual <khandual@linux.vnet.ibm.com>,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        aik@ozlabs.ru, robh@kernel.org, joe@perches.com,
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        srikar@linux.vnet.ibm.com, robin.murphy@arm.com,
        jean-philippe.brucker@arm.com, marc.zyngier@arm.com
Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices
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On Sun, Aug 05, 2018 at 11:10:15AM +1000, Benjamin Herrenschmidt wrote:
>  - One you have rejected, which is to have a way for "no-iommu" virtio
> (which still doesn't use an iommu on the qemu side and doesn't need
> to), to be forced to use some custom DMA ops on the VM side.
> 
>  - One, which sadly has more overhead and will require modifying more
> pieces of the puzzle, which is to make qemu uses an emulated iommu.
> Once we make qemu do that, we can then layer swiotlb on top of the
> emulated iommu on the guest side, and pass that as dma_ops to virtio.

Or number three:  have a a virtio feature bit that tells the VM
to use whatever dma ops the platform thinks are appropinquate for
the bus it pretends to be on.  Then set a dma-range that is limited
to your secure memory range (if you really need it to be runtime
enabled only after a device reset that rescans) and use the normal
dma mapping code to bounce buffer.