Re: [Xen-devel] Upstream Dom0 DRM problems regarding swiotlb

[Christoph Hellwig <hch@xxxxxxxxxxxxx>]

On Thu, Feb 14, 2019 at 07:03:38AM +0100, Juergen Gross wrote:
> > The thing which is different between Xen PV guests and most others (all
> > others(?), now that Lguest and UML have been dropped) is that what Linux
> > thinks of as PFN $N isn't necessarily adjacent to PFN $N+1 in system
> > physical address space.
> > 
> > Therefore, code which has a buffer spanning a page boundary can't just
> > convert a pointer to the buffer into a physical address, and hand that
> > address to a device.  You generally end up with either memory corruption
> > (DMA hitting the wrong page allocated to the guest), or an IOMMU fault
> > (DMA hitting a pages which isn't allocated to the guest).

The Linux DMA API allows for dma_map_page / dma_map_single calls to
spawn 4k boundaries.  If Xen doesn't support that it will have to bounce
buffer for that case (and get horrible performance).

But the latter text seems to agree with that.  So what is the actual
problem that started this discussion?

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