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Re: [Xen-devel] [early RFC] ARM PCI Passthrough design document

On Mon, 13 Feb 2017, Julien Grall wrote:
> Hi Stefano,
> On 10/02/17 01:01, Stefano Stabellini wrote:
> > On Fri, 3 Feb 2017, Edgar E. Iglesias wrote:
> > > A possible hack could be to allocate a chunk of DDR dedicated for PCI DMA.
> > > PCI DMA devs could be locked in to only be able to access this mem + MSI
> > > doorbell.
> > > Guests can still screw each other up but at least it becomes harder to
> > > read/write directly from each others OS memory.
> > > It may not be worth the effort though....
> > 
> > Actually, we do have the swiotlb in Dom0, which can be used to bounce
> > DMA requests over a buffer that has been previously setup to be DMA safe
> > using an hypercall. That is how the swiotlb is used on x86. On ARM it is
> > used to issue cache flushes via hypercall, but it could be adapted to do
> > both. It would degrade performance, due to the additional memcpy, but it
> > would work, I believe.
> A while ago, Globallogic suggested to use direct memory mapping for the guest
> to allow guest using DMA on platform not supporting SMMU.
> I believe we can use the same trick on platform where SMMUs can not
> distinguish PCI devices.

Yes, that would work, but only on platforms with a very limited number
of guests. However, it might still be a very common use-case on a
platform such as the Zynq MPSoC.

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