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Date: Thu, 5 Apr 2018 17:54:40 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
To: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Cc: virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, robh@kernel.org, aik@ozlabs.ru,
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        elfring@users.sourceforge.net, david@gibson.dropbear.id.au
Subject: Re: [RFC] virtio: Use DMA MAP API for devices without an IOMMU
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On Thu, Apr 05, 2018 at 08:09:30PM +0530, Anshuman Khandual wrote:
> On 04/05/2018 04:26 PM, Anshuman Khandual wrote:
> > There are certian platforms which would like to use SWIOTLB based DMA API
> > for bouncing purpose without actually requiring an IOMMU back end. But the
> > virtio core does not allow such mechanism. Right now DMA MAP API is only
> > selected for devices which have an IOMMU and then the QEMU/host back end
> > will process all incoming SG buffer addresses as IOVA instead of simple
> > GPA which is the case for simple bounce buffers after being processed with
> > SWIOTLB API. To enable this usage, it introduces an architecture specific
> > function which will just make virtio core front end select DMA operations
> > structure.
> > 
> > Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
> 
> + "Michael S. Tsirkin" <mst@redhat.com>

I'm confused by this.

static bool vring_use_dma_api(struct virtio_device *vdev)
{
        if (!virtio_has_iommu_quirk(vdev))
                return true;


Why doesn't setting VIRTIO_F_IOMMU_PLATFORM on the
hypervisor side sufficient?