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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 22 Jan 2019, hch@xxxxxxxxxxxxx wrote:
> 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.

If vring_use_dma_api is actually supposed to return true when
dma_dev->dma_mem is set, then both Peng's patch and the patch I wrote
are not fixing the real issue here.

I don't know enough about remoteproc to know where the problem actually
lies though.

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