Re: virtio DMA API?

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Mon, Aug 25, 2014 at 10:18:46AM -0700, Andy Lutomirski wrote:
> Currently, a lot of the virtio code assumes that bus (i.e. hypervisor)
> addresses are the same as physical address.  This is false on Xen, so
> virtio is completely broken.  I wouldn't be surprised if it also
> becomes a problem the first time that someone sticks a physical
> "virtio" device on a 32-bit bus on an ARM SOC with more than 4G RAM.
> 
> Would you accept patches to convert virtio_ring and virtio_pci to use
> the DMA APIs?  I think that the only real catch will be that
> virtio_ring's approach to freeing indirect blocks is currently
> incompatible with the DMA API -- it assumes that knowing the bus
> address is enough to call kfree, and I don't think that the DMA API
> provides a reverse mapping like that.

If you use the dma_map/unmap_sg all of that ends up being stuck in the
sg structure (sg->dma_address ends with the DMA addr, sg_phys(sg) gives
you the physical address).


> 
> --Andy
> 
> -- 
> Andy Lutomirski
> AMA Capital Management, LLC