Re: [Xen-devel] [RFC] virtio_ring: check dma_mem for xen_domain

["hch@infradead.org" <hch@infradead.org>]

On Tue, Jan 22, 2019 at 11:59:31AM -0800, Stefano Stabellini wrote:
> >  	if (!virtio_has_iommu_quirk(vdev))
> >  		return true;
> >  
> > @@ -260,7 +262,7 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
> >  	 * the DMA API if we're a Xen guest, which at least allows
> >  	 * all of the sensible Xen configurations to work correctly.
> >  	 */
> > -	if (xen_domain())
> > +	if (xen_domain() && !dma_dev->dma_mem)
> >  		return true;
> >  
> >  	return false;
> 
> I can see you spotted a real issue, but this is not the right fix. We
> just need something a bit more flexible than xen_domain(): there are
> many kinds of Xen domains on different architectures, we basically want
> to enable this (return true from vring_use_dma_api) only when the xen
> swiotlb is meant to be used. Does the appended patch fix the issue you
> have?

The problem generally is the other way around - if dma_dev->dma_mem
is set the device decription in the device tree explicitly requires
using this memory, so we must _always_ use the DMA API.

The problem is just that that rproc driver absuses the DMA API
in horrible ways.