linux-kernel - Re: [RFC] virtio: Use DMA MAP API for devices without an IOMMU

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Date:   Thu, 5 Apr 2018 21:34:54 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, robh@...nel.org, aik@...abs.ru,
        jasowang@...hat.com, joe@...ches.com,
        linuxppc-dev@...ts.ozlabs.org, elfring@...rs.sourceforge.net,
        david@...son.dropbear.id.au
Subject: Re: [RFC] virtio: Use DMA MAP API for devices without an IOMMU

On Fri, Apr 06, 2018 at 01:09:43AM +1000, Benjamin Herrenschmidt wrote:
> On Thu, 2018-04-05 at 17:54 +0300, Michael S. Tsirkin wrote:
> > 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@...ux.vnet.ibm.com>
> > > 
> > > + "Michael S. Tsirkin" <mst@...hat.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?
> 
> 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.


> 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.