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

["hch@xxxxxxxxxxxxx" <hch@xxxxxxxxxxxxx>]

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.

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