Linux Virtualization - Apr 2018 - [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 at
linux.vnet.ibm.com>
> > > 
> > > + "Michael S. Tsirkin" <mst at 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?
> 
> 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.

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.

On 04/06/2018 02:48 AM, Benjamin Herrenschmidt wrote:
> 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.
Also the only requirement is to bounce the I/O buffers through SWIOTLB
implemented as DMA API which the virtio core understands. There is no
need for an IOMMU to be involved for the device representation in this
case IMHO.